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Application-Layer Traffic Optimization (ALTO) Deployment Considerations

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7971.
Authors Martin Stiemerling , Sebastian Kiesel , Michael Scharf , Hans Seidel , Stefano Previdi
Last updated 2016-10-06 (Latest revision 2016-07-20)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Vijay K. Gurbani
Shepherd write-up Show Last changed 2016-05-24
IESG IESG state Became RFC 7971 (Informational)
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Consensus boilerplate Yes
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Responsible AD Mirja K├╝hlewind
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ALTO                                                      M. Stiemerling
Internet-Draft                                      Hochschule Darmstadt
Intended status: Informational                                 S. Kiesel
Expires: January 21, 2017                        University of Stuttgart
                                                               M. Scharf
                                                               H. Seidel
                                                              S. Previdi
                                                           July 20, 2016

                     ALTO Deployment Considerations


   Many Internet applications are used to access resources such as
   pieces of information or server processes that are available in
   several equivalent replicas on different hosts.  This includes, but
   is not limited to, peer-to-peer file sharing applications.  The goal
   of Application-Layer Traffic Optimization (ALTO) is to provide
   guidance to applications that have to select one or several hosts
   from a set of candidates, which are able to provide a desired
   resource.  This memo discusses deployment related issues of ALTO.  It
   addresses different use cases of ALTO such as peer-to-peer file
   sharing and CDNs and presents corresponding examples.  The document
   also includes recommendations for network administrators and
   application designers planning to deploy ALTO, such recommendations
   how to generate ALTO map information.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 21, 2017.

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  General Considerations . . . . . . . . . . . . . . . . . . . .  5
     2.1.  ALTO Entities  . . . . . . . . . . . . . . . . . . . . . .  5
       2.1.1.  Baseline Scenario  . . . . . . . . . . . . . . . . . .  5
       2.1.2.  Placement of ALTO Entities . . . . . . . . . . . . . .  7
     2.2.  Classification of Deployment Scenarios . . . . . . . . . .  8
       2.2.1.  Roles in ALTO Deployments  . . . . . . . . . . . . . .  8
       2.2.2.  Information Exposure . . . . . . . . . . . . . . . . . 11
       2.2.3.  More Advanced Deployments  . . . . . . . . . . . . . . 12
   3.  Deployment Considerations by ISPs  . . . . . . . . . . . . . . 15
     3.1.  Objectives for the Guidance to Applications  . . . . . . . 15
       3.1.1.  General Objectives for Traffic Optimization  . . . . . 15
       3.1.2.  Inter-Network Traffic Localization . . . . . . . . . . 16
       3.1.3.  Intra-Network Traffic Localization . . . . . . . . . . 17
       3.1.4.  Network Off-Loading  . . . . . . . . . . . . . . . . . 18
       3.1.5.  Application Tuning . . . . . . . . . . . . . . . . . . 19
     3.2.  Provisioning of ALTO Topology Data . . . . . . . . . . . . 20
       3.2.1.  High-Level Process and Requirements  . . . . . . . . . 20
       3.2.2.  Data Collection from Data Sources  . . . . . . . . . . 21
       3.2.3.  Partitioning and Grouping of IP Address Ranges . . . . 24
       3.2.4.  Rating Criteria and/or Cost Calculation  . . . . . . . 25
     3.3.  ALTO Focus and Scope . . . . . . . . . . . . . . . . . . . 28
       3.3.1.  Limitations of Using ALTO Beyond Design Assumptions  . 29
       3.3.2.  Limitations of Map-based Services and Potential
               Solutions  . . . . . . . . . . . . . . . . . . . . . . 30
       3.3.3.  Limitations of Non-Map-based Services and
               Potential Solutions  . . . . . . . . . . . . . . . . . 32
     3.4.  Monitoring ALTO  . . . . . . . . . . . . . . . . . . . . . 32
       3.4.1.  Impact and Observation on Network Operation  . . . . . 32
       3.4.2.  Measurement of the Impact  . . . . . . . . . . . . . . 33

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       3.4.3.  System and Service Performance . . . . . . . . . . . . 34
       3.4.4.  Monitoring Infrastructures . . . . . . . . . . . . . . 35
     3.5.  Abstract Map Examples for Different Types of ISPs  . . . . 36
       3.5.1.  Small ISP with Single Internet Uplink  . . . . . . . . 36
       3.5.2.  ISP with Several Fixed Access Networks . . . . . . . . 39
       3.5.3.  ISP with Fixed and Mobile Network  . . . . . . . . . . 40
     3.6.  Comprehensive Example for Map Calculation  . . . . . . . . 42
       3.6.1.  Example Network  . . . . . . . . . . . . . . . . . . . 42
       3.6.2.  Potential Input Data Processing and Storage  . . . . . 44
       3.6.3.  Calculation of Network Map from the Input Data . . . . 47
       3.6.4.  Calculation of Cost Map  . . . . . . . . . . . . . . . 48
     3.7.  Deployment Experiences . . . . . . . . . . . . . . . . . . 50
   4.  Using ALTO for P2P Traffic Optimization  . . . . . . . . . . . 53
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 53
       4.1.1.  Usage Scenario . . . . . . . . . . . . . . . . . . . . 53
       4.1.2.  Applicability of ALTO  . . . . . . . . . . . . . . . . 53
     4.2.  Deployment Recommendations . . . . . . . . . . . . . . . . 56
       4.2.1.  ALTO Services  . . . . . . . . . . . . . . . . . . . . 56
       4.2.2.  Guidance Considerations  . . . . . . . . . . . . . . . 57
   5.  Using ALTO for CDNs  . . . . . . . . . . . . . . . . . . . . . 60
     5.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 60
       5.1.1.  Usage Scenario . . . . . . . . . . . . . . . . . . . . 60
       5.1.2.  Applicability of ALTO  . . . . . . . . . . . . . . . . 62
     5.2.  Deployment Recommendations . . . . . . . . . . . . . . . . 63
       5.2.1.  ALTO Services  . . . . . . . . . . . . . . . . . . . . 63
       5.2.2.  Guidance Considerations  . . . . . . . . . . . . . . . 64
   6.  Other Use Cases  . . . . . . . . . . . . . . . . . . . . . . . 66
     6.1.  Application Guidance in Virtual Private Networks (VPNs)  . 66
     6.2.  In-Network Caching . . . . . . . . . . . . . . . . . . . . 68
     6.3.  Other Application-based Network Operations . . . . . . . . 69
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 70
     7.1.  ALTO as a Protocol Crossing Trust Boundaries . . . . . . . 70
     7.2.  Information Leakage from the ALTO Server . . . . . . . . . 71
     7.3.  ALTO Server Access . . . . . . . . . . . . . . . . . . . . 72
     7.4.  Faking ALTO Guidance . . . . . . . . . . . . . . . . . . . 73
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 75
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 76
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 77
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 77
     10.2. Informative References . . . . . . . . . . . . . . . . . . 77
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 81

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1.  Introduction

   Many Internet applications are used to access resources such as
   pieces of information or server processes that are available in
   several equivalent replicas on different hosts.  This includes, but
   is not limited to, peer-to-peer (P2P) file sharing applications and
   Content Delivery Networks (CDNs).  The goal of Application-Layer
   Traffic Optimization (ALTO) is to provide guidance to applications
   that have to select one or several hosts from a set of candidates,
   which are able to provide a desired resource.  The basic ideas and
   problem space of ALTO is described in [RFC5693] and the set of
   requirements is discussed in [RFC6708].  The ALTO protocol is
   specified in [RFC7285].  An ALTO server discovery procedure is
   defined in [RFC7286].

   This document discusses use cases and operational issues that can be
   expected when ALTO gets deployed.  This includes, but is not limited
   to, location of the ALTO server, imposed load to the ALTO server, and
   who initiaties the queries.  This document provides guidance on which
   ALTO services to use, and it summarizes known challenges as well as
   deployment experiences, including potential processes to generate
   ALTO network and cost maps.  It thereby complements the management
   considerations in the protocol specification [RFC7285], which are
   independent of any specific use of ALTO.

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2.  General Considerations

2.1.  ALTO Entities

2.1.1.  Baseline Scenario

   The ALTO protocol [RFC7285] is a client/server protocol, operating
   between a number of ALTO clients and an ALTO server, as sketched in
   Figure 1.  Below, the baseline deployment scenario for ALTO entities
   is first reviewed independently of the actual use case.  Specific
   examples are then discussed in the remainder of this document.

                 |  ALTO    |
                 |  Server  |
            ,-''       |       `--.
          ,'           |           `.
         (     Network |             )
          `.           |           ,'
            `--.       |       _.-'
    +----------+  +----------+   +----------+
    |  ALTO    |  |  ALTO    |...|  ALTO    |
    |  Client  |  |  Client  |   |  Client  |
    +----------+  +----------+   +----------+

        Figure 1: Baseline deployment scenario of the ALTO protocol

   This document uses the terminology introduced in [RFC5693].  In
   particular, the following terms are defined by [RFC5693]:

   o  ALTO Service: Several resource providers may be able to provide
      the same resource.  The ALTO service gives guidance to a resource
      consumer and/or resource directory about which resource
      provider(s) to select in order to optimize the client's
      performance or quality of experience, while improving resource
      consumption in the underlying network infrastructure.

   o  ALTO Server: A logical entity that provides interfaces to satisfy
      the queries about a particular ALTO service.

   o  ALTO Client: The logical entity that sends ALTO queries.
      Depending on the architecture of the application, one may embed it

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      in the resource consumer and/or in the resource directory.

   o  Resource Consumer: For P2P applications, a resource consumer is a
      specific peer that needs to access resources.  For client-server
      or hybrid applications, a consumer is a client that needs to
      access resources.

   We differentiate between an ALTO Client and a Resource Consumer as
   follows: The resource consumer is specific instance of a software
   ("process") running on a specific host.  It is a client instance of a
   client/server application or a peer of a peer-to-peer application.
   It is the given (constant) endpoint of the data transmissions to be
   optimized using ALTO.  The optimization is done by wisely choosing
   the other ends of these data flows (i.e., the server(s) in a client/
   server application or the peers in a peer-to-peer application), using
   guidance from ALTO and possibly other information.  An ALTO client is
   a piece of software (e.g., a software library) that implements the
   client entity of the ALTO protocol as specified in [RFC7285].  It
   consists of data structures that are suitable for representing ALTO
   queries, replies, network and cost maps, etc.  Furthermore, it has to
   implement an HTTP client and a JSON encoder/decoder, or it has to
   include other software libraries that provide these building blocks.
   In the simplest case, this ALTO client library can be linked (or
   otherwise incorporated) into the resource consumer, in order to
   retrieve information from an ALTO server and thereby satisfy the
   resource consumer's need for guidance.  However, other configurations
   are possible as well, as discussed in Section 2.1.2 and other
   sections of this document.

   This document uses the term "Resource Directory" as defined in
   [RFC5693], i.e., to denote an entity that is logically separate from
   the resource consumer and that assists the resource consumer to
   identify a set of resource providers (e.g., a tracker in a peer-to-
   peer application).  This term and its meaning is not to be confused
   with the "Information Resource Directory (IRD)" defined as a part of
   the ALTO protocol [RFC7285], i.e., a list of available information
   resources offered by a specific ALTO service and the URIs at which
   each can be accessed.  For the remainder of this document, the term
   Resource Directory is to be interpreted as defined in [RFC5693].

   According to these definitions, both an ALTO server and an ALTO
   client are logical entities.  A particular ALTO service may be
   offered by more than one ALTO server.  In ALTO deployments, the
   functionality of an ALTO server can therefore be realized by several
   server instances, e.g., by using load balancing between different
   physical servers.  The term ALTO server should not be confused with
   use of a single physical server.

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2.1.2.  Placement of ALTO Entities

   The ALTO server and ALTO clients may be situated at various places in
   a network topology.  An importent differentiation is whether the ALTO
   client is located on the host that is the endpoint of the data
   transmissions to be optimized with ALTO (see Figure 2), or whether
   the ALTO client is located on a resource directory, which assists
   peers or clients in finding other peers or servers, respectively, but
   does not directly take part in the data transmission (see Figure 3).

                                              |     App      |
                                              +-----------+  |
                                          ===>|ALTO Client|  |****
                                       ===    +-----------+--+   *
                                    ===                    *     *
                                 ===                       *     *
      +-------+     +-------+<===             +--------------+   *
      |       |     |       |                 |     App      |   *
      |       |.....|       |<========        +-----------+  |   *
      |       |     |       |        ========>|ALTO Client|  |   *
      +-------+     +-------+<===             +-----------+--+   *
      Source of       ALTO       ==                        *     *
      topological    Server        ==                      *     *
      information                    ==       +--------------+   *
                                       ==     |     App      |   *
                                         ==   +-----------+  |****
                                           ==>|ALTO Client|  |
      === ALTO protocol
      *** Application protocol
      ... Provisioning protocol

     Figure 2: Overview of protocol interaction between ALTO elements
                       without a resource directory

   Figure 2 shows the operational model for an ALTO client running at
   endpoints.  An example would be a peer-to-peer file sharing
   application that does not use a tracker, such as edonkey.  In
   addition, ALTO clients at peers could also be used in a similar way
   even if there is a tracker, as further discussed in Section 4.1.2.

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                                                     **| App |****
                                                   **  +-----+   *
                                                 **       *      *
                                               **         *      *
      +-------+     +-------+     +--------------+        *      *
      |       |     |       |     |              |     +-----+   *
      |       |.....|       |     +-----------+  |*****| App |   *
      |       |     |       |<===>|ALTO Client|  |     +-----+   *
      +-------+     +-------+     +-----------+--+        *      *
      Source of       ALTO          Resource   **         *      *
      topological    Server         directory    **       *      *
      information                                  **  +-----+   *
                                                     **| App |****
      === ALTO protocol
      *** Application protocol
      ... Provisioning protocol

   Figure 3: Overview of protocol interaction between ALTO elements with
                           a resource directory

   In Figure 3, a use case with a resource directory is illustrated,
   e.g., a tracker in a peer-to-peer file-sharing application such as
   BitTorrent.  Both deployment scenarios may differ in the number of
   ALTO clients that access an ALTO service.  If an ALTO client is
   implemented in a resource directory, an ALTO server may be accessed
   by a limited and less dynamic set of clients, whereas in the general
   case any host could be an ALTO client.  This use case is further
   detailed in Section 4.

   Using ALTO in CDNs may be similar to a resource directory
   [I-D.jenkins-alto-cdn-use-cases].  The ALTO server can also be
   queried by CDN entities to get guidance about where a particular
   client accessing data in the CDN is located in the Internet Service
   Provider's network, as discussed in Section 5.

2.2.  Classification of Deployment Scenarios

2.2.1.  Roles in ALTO Deployments

   ALTO is a general-purpose protocol and it is intended to be used by a
   wide range of applications.  In different use cases, applications,
   resource directories, etc. can correspond to different functionality.
   The use cases listed in this document are not meant to be
   comprehensive.  This also implies that there are different

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   possibilities where the ALTO entities are actually located, i.e., if
   the ALTO clients and the ALTO server are in the same Internet Service
   Provider (ISP) domain, or if the clients and the ALTO server are
   managed/owned/located in different domains.

   An ALTO deployment involves four kinds of entities:

   1.  Source of topological information

   2.  ALTO server

   3.  ALTO client

   4.  Resource consumer

   Each of these entities corresponds to a certain role, which results
   in requirements and constraints on the interaction between the

   A key design objective of the ALTO service is that each of these four
   roles can be separated, i.e., they can be realized by different
   organizations or disjoint system components.  ALTO is inherently
   designed for use in multi-domain environments.  Most importantly,
   ALTO is designed to enable deployments in which the ALTO server and
   the ALTO client are not located within the same administrative

   As explained in [RFC5693], from this follows that at least three
   different kinds of entities can operate an ALTO server:

   1.  Network operators.  Network Service Providers (NSPs) such as
       Internet Service Providers (ISPs) may have detailed knowledge of
       their network topology and policies.  In this case, the source of
       the topology information and the provider of the ALTO server may
       be part of the same organization.

   2.  Third parties.  Topology information could also be collected by
       companies or organizations that are distinct from the network
       operators, yet have arranged certain legal agreements with one or
       more network operators, regarding access to their topology
       information and/or doing measurements in their networks.
       Examples of such entities could be Content Delivery Network (CDN)
       operators or companies specialized on offering ALTO services on
       behalf of ISPs.

   3.  User communities.  User communities could run distributed
       measurements for estimating the topology of the Internet.  In
       this case the topology information may not originate from ISP

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   Regarding the interaction between ALTO server and client, ALTO
   deployments can be differentiated according to the following aspects:

   1.  Applicable trust model: The deployment of ALTO can differ
       depending on whether ALTO client and ALTO server are operated
       within the same organization and/or network, or not.  This
       affects a number of constraints, because the trust model is very
       different.  For instance, as discussed later in this memo, the
       level-of-detail of maps can depend on who the involved parties
       actually are.

   2.  Composition of the user group: The main use case of ALTO is to
       provide guidance to any Internet application.  However, an
       operator of an ALTO server could also decide to offer guidance
       only to a set of well-known ALTO clients, e. g., after
       authentication and authorization.  In the peer-to-peer
       application use case, this could imply that only selected
       trackers are allowed to access the ALTO server.  The security
       implications of using ALTO in closed groups differ from the
       public Internet.

   3.  Covered destinations: In general, an ALTO server has to be able
       to provide guidance for all potential destinations.  Yet, in
       practice a given ALTO client may only be interested in a subset
       of destinations, e.g., only in the network cost between a limited
       set of resource providers.  For instance, CDN optimization may
       not need the full ALTO cost maps, because traffic between
       individual residential users is not in scope.  This may imply
       that an ALTO server only has to provide the costs that matter for
       a given user, e. g., by customized maps.

   The following sections enumerate different classes of use cases for
   ALTO, and they discuss deployment implications of each of them.  An
   ALTO server can in principle be operated by any organization, and
   there is no requirement that an ALTO server is deployed and operated
   by an ISP.  Yet, since the ALTO solution is designed for ISPs, most
   examples in this document assume that the operator of an ALTO server
   is a network operator (e.g., an ISP or the network department in a
   large enterprise) that offers ALTO guidance in particular to users of
   this network.

   It must be emphasized that any application using ALTO must also work
   if no ALTO servers can be found or if no responses to ALTO queries
   are received, e.g., due to connectivity problems or overload
   situations (see also [RFC6708]).

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2.2.2.  Information Exposure

   There are basically two different approaches how an ALTO server can
   provide network information and guidance:

   1.  The ALTO server provides maps that contain provider-defined cost
       values between network location groupings (e.g., sets of IP
       prefixes).  These maps can be retrieved by clients via the ALTO
       protocol, and the actual processing of the map data is done
       inside the client.  Since the maps contain (aggregated) cost
       information for all endpoints, the client does not have to reveal
       any internal operational data, such as the IP addresses of
       candidate resource providers.  The ALTO protocol supports this
       mode of operation by the Network and Cost Map Service.

   2.  The ALTO server provides a query interface that returns costs or
       rankings for explicitly specified endpoints.  This means that the
       query of the ALTO client has to include additional information
       (e.g., a list of IP addresses).  The server then calculates and
       returns costs or rankings for the endpoints specified in the
       request (e.g., a sorted list of the IP addresses).  In ALTO, this
       approach can be realized by the Endpoint Cost Service and other
       related services.

   Both approaches have different privacy implications for the server
   and client:

   For the client, approach 1 has the advantage that all operational
   information stays within the client and is not revealed to the
   provider of the server.  However, this service implies that a network
   operator providing an ALTO server has to expose a certain amount of
   information about its network structure (e.g., IP prefixes or
   topology information in general).

   For the operator of a server, approach 2 has the advantage that the
   query responses reveal less topology information to ALTO clients.
   However, it should be noted that collaborating ALTO clients could
   gather more information than expected by aggregating and correlating
   responses to multiple queries sent to the ALTO server (see Section
   5.2.1, item (3) of [RFC6708]).  Furthermore, this method requires
   that clients send internal operational information to the server,
   such as the IP addresses of hosts also running the application.  For
   clients, such data can be sensitive.

   As a result, both approaches have their pros and cons, as further
   detailed in Section 3.3.

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2.2.3.  More Advanced Deployments

   From an ALTO client's perspective, there are different ways to use

   1.  Single service instance with single metric guidance: An ALTO
       client only obtains guidance regarding a single metric (e.g.,
       "routingcost") from a single ALTO service, e.g., an ALTO server
       that is offered by the network service provider of the
       corresponding access network.  Corresponding ALTO server
       instances can be discovered e.g. by ALTO server discovery
       [RFC7286] [I-D.kiesel-alto-xdom-disc].  Since the ALTO protocol
       is an HTTP-based, REST-ful protocol, the operator of an ALTO may
       use well-known techniques for serving large web sites, such as
       load balancers, in order to serve a large number of ALTO queries.
       The ALTO protocol also supports the use of different URIs for
       different ALTO features and thereby the distribution of the
       service onto several servers.

   2.  Single service instance with multiple metric guidance: An ALTO
       client could also query an ALTO service for different kinds of
       information, e.g., cost maps with different metrics.  The ALTO
       protocol is extensible and permits such operation.  However, ALTO
       does not define how a client shall deal with different forms of
       guidance, and it is up to the client to interpret the received
       information accordingly.

   3.  Multiple service instances: An ALTO client can also decide to
       access multiple ALTO servers providing guidance, possibly from
       different operators or organizations.  Each of these services may
       only offer partial guidance, e.g., for a certain network
       partition.  In that case, it may be difficult for an ALTO client
       to compare the guidance from different services.  Different
       organization may use different methods to determine maps, and
       they may also have different (possibly even contradicting or
       competing) guidance objectives.  How to discover multiple ALTO
       servers and how to deal with conflicting guidance is an open

   There are also different options regarding the synchronization of
   guidance offered by an ALTO service:

   1.  Authoritative servers: An ALTO server instance can provide
       guidance for all destinations for all kinds of ALTO clients.

   2.  Cascaded servers: An ALTO server may itself include an ALTO
       client and query other ALTO servers, e.g., for certain
       destinations.  This results is a cascaded deployment of ALTO

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       servers, as further explained below.

   3.  Inter-server synchronization: Different ALTO servers may
       communicate by other means.  This approach is not further
       discussed in this document.

   An assumption of the ALTO design is that ISPs operate ALTO servers
   independently, irrespectively of other ISPs.  This may be true for
   most envisioned deployments of ALTO but there may be certain
   deployments that may have different settings.  Figure 4 shows such
   setting with a university network that is connected to two upstream
   providers.  NREN is a National Research and Education Network, which
   provides cheap high-speed connectivity to specific destinations,
   e.g., other universities.  ISP is a commercial upstream provider from
   which the university buys connectivity to all destinations that
   cannot be reached via the NREN.  The university, as well as ISP, are
   operating their own ALTO server.  The ALTO clients, located on the
   peers in the university network will contact the ALTO server located
   at the university.

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          |    ISP    |
          |   ALTO    |<==========================++
          |  Server   |                           ||
          +-----------+                           ||
            ,-------.            ,------.         ||
         ,-'         `-.      ,-'         `-.     ||
        /   Commercial  \    /               \    ||
       (    Upstream     )  (       NREN      )   ||
        \   ISP         /    \               /    ||
         `-.         ,-'      `-.         ,-'     ||
            `---+---'            `+------'        ||
                |                 |               ||
                |                 |               ||
                |,-------------.  |               \/
              ,-+               `-+          +-----------+
            ,'      University     `.        |University |
           (        Network          )       |   ALTO    |
            `.                      /        |  Server   |
              `-.               +--'         +-----------+
                 `+------------'|              /\     /\
                  |             |              ||     ||
         +--------+-+         +-+--------+     ||     ||
         |   Peer1  |         |   PeerN  |<====++     ||
         +----------+         +----------+            ||
              /\                                      ||
              ||                                      ||

      === ALTO protocol

                Figure 4: Example of a cascaded ALTO server

   In this setting, all destinations that can be reached via the NREN
   are preferred in the rating of the university's ALTO server.  In
   contrast, all traffic that is not routed via the NREN will be handled
   by the commercial upstream ISP and is in general less preferred due
   to the associated costs.  Yet, there may be significant differences
   between various destinations reached via the ISP.  Therefore, the
   ALTO server at the university may also include the guidance given by
   the ISP ALTO server in its replies to the ALTO clients.  This is an
   example for cascaded ALTO servers.

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3.  Deployment Considerations by ISPs

3.1.  Objectives for the Guidance to Applications

3.1.1.  General Objectives for Traffic Optimization

   The Internet consists of many networks.  The networks are owned and
   managed by different network operators, such as commercial Internet
   Service Providers (ISPs), enterprise IT departments, universities,
   and other organizations.  These network operators provide network
   connectivity, e.g., by access networks, such as cable networks, xDSL
   networks, 3G/4G mobile networks, etc.  Network operators need to
   manage, to control and to audit the traffic.  Therefore, it is
   important to understand how to deploy an ALTO service and its
   expected impact.

   The general objective of ALTO is to give guidance to applications on
   what endpoints (e.g., IP addresses or IP prefixes) are to be
   preferred according to the operator of the ALTO server.  The ALTO
   protocol gives means to let the ALTO server operator express its
   preference, whatever this preference is.

   ALTO enables network operators to support application-level traffic
   engineering by influencing application resource provider selection.
   This traffic engineering can have different objectives:

   1.  Inter-network traffic localization: ALTO can help to reduce
       inter-domain traffic.  The networks of different network
       operators are interconnected through peering points.  From a
       business view, the inter-network settlement is needed for
       exchanging traffic between these networks.  These peering
       agreements can be costly.  To reduce these costs, a simple
       objective is to decrease the traffic exchange across the peering
       points and thus keep the traffic in the own network or Autonomous
       System (AS) as far as possible.

   2.  Intra-network traffic localization: In case of large network
       operators, the network may be grouped into several networks,
       domains, or Autonomous Systems (ASs).  The core network includes
       one or several backbone networks, which are connected to multiple
       aggregation, metro, and access networks.  If traffic can be
       limited to certain areas such as access networks, this decreases
       the usage of backbone and thus helps to save resources and costs.

   3.  Network off-loading: Compared to fixed networks, mobile networks
       have some special characteristics, including lower link
       bandwidth, high cost, limited radio frequency resource, and
       limited terminal battery.  In mobile networks, wireless links

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       should be used efficiently.  For example, in the case of a P2P
       service, it is likely that hosts should prefer retrieving data
       from hosts in fixed networks, and avoid retrieving data from
       mobile hosts.

   4.  Application tuning: ALTO is also a tool to optimize the
       performance of applications that depend on the network and
       perform resource provider selection decisions among network
       endpoints.  And example is the network-aware selection of Content
       Delivery Network (CDN) caches.

   In the following, these objectives are explained in more detail with

3.1.2.  Inter-Network Traffic Localization

   ALTO guidance can be used to keep traffic local in a network, for
   instance in order to reduce peering costs.  An ALTO server can let
   applications prefer other hosts within the same network operator's
   network instead of randomly connecting to other hosts that are
   located in another operator's network.  Here, a network operator
   would always express its preference for hosts in its own network,
   while hosts located outside its own network are to be avoided (i.e.,
   they are undesired to be considered by the applications).  Figure 5
   shows such a scenario where hosts prefer hosts in the same network
   (e.g., Host 1 and Host 2 in ISP1 and Host 3 and Host 4 in ISP2).

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                            ,-------.         +-----------+
          ,---.          ,-'         `-.      |   Host 1  |
       ,-'     `-.      /     ISP 1   ########|ALTO Client|
      /           \    /              #  \    +-----------+
     /    ISP X    \   |              #  |    +-----------+
    /               \  \              ########|   Host 2  |
   ;             +----------------------------|ALTO Client|
   |             |   |   `-.         ,-'      +-----------+
   |             |   |      `-------'
   |     Inter-  |   |      ,-------.         +-----------+
   :     network |   ;   ,-'         `########|   Host 3  |
    \    traffic |  /   /     ISP 2   # \     |ALTO Client|
     \           | /   /              #  \    +-----------+
      \          |/    |              #  |    +-----------+
       `-.     ,-|     \              ########|   Host 4  |
          `---'  +----------------------------|ALTO Client|
                         `-.         ,-'      +-----------+

       ### preferred "connections"
       --- non-preferred "connections"

               Figure 5: Inter-network traffic localization

   Examples for corresponding ALTO maps can be found in Section 3.5.
   Depending on the application characteristics, it may not be possible
   or not even desirable to completely localize all traffic.

3.1.3.  Intra-Network Traffic Localization

   The previous section describes the results of the ALTO guidance on an
   inter-network level.  In the same way, ALTO can also be used for
   intra-network localization.  In this case, ALTO provides guidance on
   which internal hosts are to be preferred inside a single network
   (e.g., one AS).  This application-level traffic engineering can
   reduce the capacity requirements in the core network of an ISP.
   Figure 6 shows such a scenario where Host 1 and Host 2 are located in
   an access net 1 of ISP 1 and connect via a low capacity link to the
   core of the same ISP 1.  If Host 1 and Host 2 exchange their data
   with remote hosts, they would probably congest the bottleneck link.

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              Bottleneck    ,-------.         +-----------+
          ,---.     |    ,-'         `-.      |   Host 1  |
       ,-'     `-.  |   /     ISP 1   ########|ALTO Client|
      /           \ |  /    (Access   #  \    +-----------+
     /    ISP 1    \|  |     net 1)   #  |    +-----------+
    /   (Core       V  \              ########|   Host 2  |
   ;    network) +--X~~~X---------------------|ALTO Client|
   |             |   |   `-.         ,-'      +-----------+
   |             |   |      `-------'
   |             |   |      ,-------.         +-----------+
   :             |   ;   ,-'         `########|   Host 3  |
    \            |  /   /     ISP 1   # \     |ALTO Client|
     \           | /   /     (Access  #  \    +-----------+
      \          |/    |      net 2)  #  |    +-----------+
       `-.     ,-X     \              ########|   Host 4  |
          `---'  ~~~~~~~X---------------------|ALTO Client|
                   ^     `-.         ,-'      +-----------+
                   |        `-------'
       ### preferred "connections"
       --- non-preferred "connections"

               Figure 6: Intra-network traffic localization

   The operator can guide the hosts in such a situation to try first
   local hosts in the same network islands, avoiding or at least
   lowering the effect on the bottleneck link, as shown in Figure 6.

   The objective is to avoid bottlenecks by optimized endpoint selection
   at application level.  That said, it must be understood that ALTO is
   not a general purpose method to deal with the congestion at the

3.1.4.  Network Off-Loading

   Another scenario is off-loading traffic from networks.  This use of
   ALTO can be beneficial in particular in mobile networks.  A network
   operator may have the desire to guide hosts in its mobile network to
   use hosts outside this mobile network.  One reason could be that the
   wireless network or the mobile hosts were not designed for direct
   peer-to-peer communications between mobile hosts, and therefore, it
   makes sense for peers to fetch content from remote peers in other
   parts of the Internet.

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                            ,-------.         +-----------+
          ,---.          ,-'         `-.      |   Host 1  |
       ,-'     `-.      /     ISP 1   +-------|ALTO Client|
      /           \    /    (Mobile   |  \    +-----------+
     /    ISP X    \   |    network)  |  |    +-----------+
    /               \  \              +-------|   Host 2  |
   ;             #############################|ALTO Client|
   |             #   |   `-.         ,-'      +-----------+
   |             #   |      `-------'
   |             #   |      ,-------.
   :             #   ;   ,-'         `-.
    \            #  /   /     ISP 2     \
     \           # /   /     (Fixed      \
      \          #/    |     network)    |    +-----------+
       `-.     ,-#     \                 /    |   Host 3  |
          `---'  #############################|ALTO Client|
                         `-.         ,-'      +-----------+

       ### preferred "connections"
       --- non-preferred "connections"

              Figure 7: ALTO traffic network de-localization

   Figure 7 shows the result of such a guidance process where Host 2
   prefers a connection with Host 3 instead of Host 1, as shown in
   Figure 5.

   A realization of this scenario may have certain limitations and may
   not be possible in all cases.  For instance, it may require the ALTO
   server to distinguish mobile and non-mobile hosts based on their IP
   address.  This may depend on mobility solutions and may not be
   possible or accurate.  In general, ALTO is not intended as a fine-
   grained traffic engineering solution for individual hosts.  Instead,
   it typically works on aggregates (e.g., if it is known that certain
   IP prefixes are often assigned to mobile users).

3.1.5.  Application Tuning

   ALTO can also provide guidance to optimize the application-level
   topology of networked applications, e.g., by exposing network
   performance information.  Applications can often run their own
   measurements to determine network performance, e.g., by active delay
   measurements or bandwidth probing, but such measurements result in
   overhead and complexity.  Accessing an ALTO server can be a simpler
   alternative.  In addition, an ALTO server may also expose network
   information that applications cannot easily measure or reverse-

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3.2.  Provisioning of ALTO Topology Data

3.2.1.  High-Level Process and Requirements

   A process to generate ALTO topology information typically comprises
   several steps.  The first step is to gather information, which is
   described in the following section.  The subsequent sections then
   describe how the gathered data can be processed, and which methods
   can be applied to generate the information exposed by ALTO, such as
   network and cost maps.

   Providing ALTO guidance can result in a win-win situation both for
   network providers and users of the ALTO information.  Applications
   possibly get a better performance, while the network provider has
   means to optimize the traffic engineering and thus its costs.  Yet,
   there can be security concerns with exposing topology data.
   Corresponding limitations are discussed in Section 7.2.

   ISPs may have important privacy requirements when deploying ALTO,
   which have to be taken into account when processing ALTO topology
   data.  In particular, an ISP may not be willing to expose sensitive
   operational details of its network.  The topology abstraction of ALTO
   enables an ISP to expose the network topology at a desired
   granularity only, determined by security policies.

   With the Endpoint Cost Service (ECS), the ALTO client does not have
   to implement any specific algorithm or mechanism in order to
   retrieve, maintain and process network topology information (of any
   kind).  The complexity of the network topology (computation,
   maintenance and distribution) is kept in the ALTO server and ECS is
   delivered on demand.  This allows the ALTO server to enhance and
   modify the way the topology information sources are used and
   combined.  This simplifies the enforcement of privacy policies of the

   The ALTO Network Map and Cost Map service expose an abstract view on
   the ISP network topology.  Therefore, care is needed when
   constructing those maps in order to take privacy policies into
   account, as further discussed in Section 3.2.3.  The ALTO protocol
   also supports further features such as endpoint properties, which
   could also be used to expose topology guidance.  The privacy
   considerations for ALTO maps also apply to such ALTO extensions.

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3.2.2.  Data Collection from Data Sources

   The first step in the process of generating ALTO information is to
   gather the required information from the network.  An ALTO server can
   collect topological information from a variety of sources in the
   network and provides a cohesive, abstract view of the network
   topology to applications using an ALTO client.  Topology data sources
   may include routing protocols, network policies, state and
   performance information, geo-location, etc.  An ALTO server requires
   at least some topology and/or routing information, i.e., information
   about existing endpoints and their interconnection.  With this
   information it is in principle possible to compute paths between all
   known endpoints.  Based on such basic data, the ALTO server builds an
   ALTO-specific network topology that represents the network as it
   should be understood and utilized by applications (resource
   consumers) at endpoints using ALTO services (e.g., Network/Cost Map
   Service or ECS).  A basic dataset can be extended by many other
   information obtainable from the network.

   The ALTO protocol does not assume a specific network technology or
   topology.  In principle, ALTO can be used with various types of
   addresses (Endpoint Addresses).  [RFC7285] defines the use of IPv4/
   IPv6 addresses or prefixes in ALTO, but further address types could
   be added by extensions.  In this document, only the use of IPv4/IPv6
   addresses is considered.

   The exposure of network topology information is controlled and
   managed by the ALTO server.  ALTO abstract network topologies can be
   automatically generated from the physical or logical topology of the
   network, e.g., using "live" network data.  The generation would
   typically be based on policies and rules set by the network operator.
   The maps and the guidance can significantly differ depending on the
   use case, the network architecture, and the trust relationship
   between ALTO server and ALTO client, etc.  Besides the security
   requirements that consist of not delivering any confidential or
   critical information about the infrastructure, there are efficiency
   requirements in terms of what aspects of the network are visible and
   required by the given use case and/or application.

   The ALTO server operator has to ensure that the ALTO topology does
   not reveal any details that would endanger the network integrity and
   security.  For instance, ALTO is not intended to leak raw Interior
   Gateway Protocol (IGP) or Border gateway Protocol (BGP) databases to
   ALTO clients.

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                 +--------+   +--------+
                 |  ALTO  |   |  ALTO  |
                 | Client |   | Client |
                 +--------+   +--------+
                        /\     /\
                        ||     || ALTO protocol
                        ||     ||
                        \/     \/
                       |  ALTO   |
                       | Server  |
                        : :   : :
                        : :   : :
             +..........+ :   : +..........+ Provisioning
             :            :   :            : protocol
             :            :   :            :
     +---------+ +---------+ +---------+ +---------+
     |   BGP   | |   I2RS  | |   PCE   | |   NMS   | Potential
     | Speaker | |  Client | |         | |   OSS   | data sources
     +---------+ +---------+ +---------+ +---------+
          ^           ^           ^           ^
          |           |           |           |
     Link-State     I2RS         TED     Topology and traffic related
      NLRI for      data         data    data from SNMP, NETCONF,
      IGP/BGP                            RESTCONF, REST, IPFIX, etc.

                 Figure 8: Potential data sources for ALTO

   As illustrated in Figure 8, the topology data used by an ALTO server
   can originate from different data sources:

   o  Relevant information sources are interior gateway protocols (IGPs)
      or the Border Gateway Protocol (BGP).  An ALTO server could get
      network routing information by listening to IGPs and/or peering
      with BGP speakers.  For data collection, link-state protocols are
      more suitable since every router propagates its information
      throughout the whole network.  Hence, it is possible to obtain
      information about all routers and their neighbors from one single
      router in the network.  In contrast, distance-vector protocols are
      less suitable since routing information is only shared among
      neighbors.  To obtain the whole topology with distance-vector
      routing protocols it is necessary to retrieve routing information
      from every router in the network.

   o  The document [RFC7752] describes a mechanism by which link-state
      and traffic engineering information can be collected from networks
      and shared with external components using the BGP routing

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      protocol.  This is achieved using a new BGP Network Layer
      Reachability Information (NLRI) encoding format.  The mechanism is
      applicable to physical and virtual IGP links and can also include
      Traffic Engineering (TE) data.  For instance, prefix data can be
      carried and originated in BGP, while TE data is originated and
      carried in an IGP.  The mechanism described is subject to policy

   o  The Interface to the Routing System (I2RS) is a solution for state
      transfer in and out of the Internet's routing system
      [I-D.ietf-i2rs-architecture].  An ALTO server could use an I2RS
      client to observe routing-related information.  With the rise of
      Software-Defined Networking (SDN) and a decoupling of network data
      and control plane, topology information could also be fetched from
      an SDN controller.  If I2RS is used, [RFC7922] provides
      traceability for these interactions.  This scenario is not further
      discussed in the remainder of this document.

   o  Another potential source of topology information could be a Path
      Computation Element (PCE) [RFC4655].  Topology and traffic related
      information can be retrieved from the the Traffic Engineering
      Database (TED) and Label Switched Path Database (LSP-DB).  This
      scenario is not further discussed in the remainder of this

   o  An ALTO server can also leverage a Network Management System (NMS)
      or an Operations Support System (OSS) as data sources.  NMS or OSS
      solutions are used to control, operate, and manage a network,
      e.g., using the Simple Network Management Protocol (SNMP) or
      NETCONF.  As explained for instance in [RFC7491], the NMS and OSS
      can be consumers of network events reported and can act on these
      reports as well as displaying them to users and raising alarms.
      In addition, NMS and OSS systems may have access to routing
      information and network inventory data (e.g., links, nodes, or
      link properties not visible to routing protocols, such as Shared
      Risk Link Groups).  Furthermore, Operations, Administration, and
      Maintenance (OAM) information can be leveraged, including traffic
      utilization obtained from IPFIX, event notifications (e.g., via
      syslog), liveness detection (e.g., bidirectional forwarding
      detection, BFD).  NMS or OSS systems also may have functions to
      correlate and orchestrate information originating from other data
      sources.  For instance, it could be required to correlate IP
      prefixes with routers (Provider, Provider Edge, Customer Edge,
      etc.), IGP areas, VLAN IDs, or policies.

   In the context of the provisioning protocol, topology information
   could be modeled in a YANG data model

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   The data sources mentioned so far are only a subset of potential
   topology sources and protocols.  Depending on the network type, (e.g.
   mobile, satellite network) different hardware and protocols are in
   operation to form and maintain the network.

   In general it is challenging to gather detailed information about the
   whole Internet, since the network consists of multiple domains and in
   many cases it is not possible to collect information across network
   borders.  Hence, potential information sources may be limited to a
   certain domain.

3.2.3.  Partitioning and Grouping of IP Address Ranges

   ALTO introduces provider-defined network location identifiers called
   Provider-defined Identifiers (PIDs) to aggregate network endpoints in
   the Map Services.  Endpoints within one PID may be treated as single
   entity, assuming proximity based on network topology or other
   similarity.  A key use case of PIDs is to specify network preferences
   (costs) between PIDs instead of individual endpoints.  It is up to
   the operator of the ALTO server how to group endpoints and how to
   assign PIDs.  For example, a PID may denote a subnet, a set of
   subnets, a metropolitan area, a POP, an autonomous system, or a set
   of autonomous systems.

   This document only considers deployment scenarios in which PIDs
   expand to a set of IP address ranges (CIDR).  A PID is characterized
   by a string identifier and its associated set of endpoint addresses
   [RFC7285].  If an ALTO server offers the Map Service, corresponding
   identifiers have to be configured.

   An automated ALTO implementation may use dynamic algorithms to
   aggregate network topology.  However, it is often desirable to have a
   mechanism through which the network operator can control the level
   and details of network aggregation based on a set of requirements and
   constraints.  This will typically be governed by policies that
   enforce a certain level of abstraction and prevent leakage of
   sensitive operational data.

   For instance, an ALTO server may leverage BGP information that is
   available in a networks service provider network layer and compute
   the group of prefix.  An example are BGP communities, which are used
   in MPLS/IP networks as a common mechanism to aggregate and group
   prefixes.  A BGP community is an attribute used to tag a prefix to
   group prefixes based on mostly any criteria (as an example, most ISP
   networks originate BGP prefixes with communities identifying the
   Point of Presence (PoP) where the prefix has been originated).  These
   BGP communities could be used to map IP address ranges to PIDs.  By
   an additional policy, the ALTO server operator may decide an

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   arbitrary cost defined between groups.  Alternatively, there are
   algorithms that allow the dynamic computation of costs between
   groups.  The ALTO protocol itself is independent of such algorithms
   and policies.

3.2.4.  Rating Criteria and/or Cost Calculation

   An ALTO server indicates preferences amongst network locations in the
   form of abstract costs.  These costs are generic costs and can be
   internally computed by the operator of the ALTO server according to
   its own policy.  For a given ALTO network map, an ALTO cost map
   defines directional costs pairwise amongst the set of source and
   destination network locations defined by the PIDs.

   The ALTO protocol permits the use of different cost types.  An ALTO
   cost type is defined by the combination of a cost metric and a cost
   mode.  The cost metric identifies what the costs represent.  The cost
   mode identifies how the costs should be interpreted, i.e., whether
   returned costs should be interpreted as numerical values or ordinal
   rankings.  The ALTO protocol also allows the definition of additional
   constraints defining which elements of a cost map shall be returned.

   The ALTO protocol specification [RFC7285] defines the "routingcost"
   cost metric as the basic set of rating criteria, which has to be
   supported by all implementations.  This cost metric conveys a generic
   measure for the cost of routing traffic from a source to a
   destination.  A lower value indicates a higher preference for traffic
   to be sent from a source to a destination.  How that metric is
   calculated is up to the ALTO server.

   It is possible to calculate the "routingcost" cost metric based on
   actual routing protocol information.  Typically, Interior Gateway
   Protocols (IGP) provide details about endpoints and links within a
   given network, while the Bordger Gateway Protocol (BGPs) is used to
   provide details about links to endpoints in other networks.  Besides
   topology and routing information, networks have a multitude of other
   attributes about their state, condition, and operation.  That
   comprises but is not limited to attributes like link utilization,
   bandwidth and delay, ingress/egress points of data flows from/towards
   endpoints outside of the network up to the location of nodes and

   In order to enable use of extended information, there is a protocol
   extension procedure to add new ALTO cost types.  The following list
   gives an overview on further rating criteria that have been proposed
   or which are in use by ALTO-related prototype implementations.  This
   list is not intended as normative text.  Instead, its only purpose is
   to document and discuss rating criteria that have been proposed so

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   far.  Whether such rating criteria are useful and whether the
   corresponding information would actually be made available by ISPs
   can also depend on the use case of ALTO.  A definition of further
   metrics can be found for instance in [I-D.wu-alto-te-metrics].

   Distance-related rating criteria:

   o  Relative topological distance: The term relative means that a
      larger numerical value means greater distance, but it is up to the
      ALTO service how to compute the values, and the ALTO client will
      not be informed about the nature of the computation.  One way to
      determine relative topological distance may be counting AS hops,
      but when querying this parameter, the ALTO client must not assume
      that the numbers actually are AS hops.  In addition to the AS
      path, a relative cost value could also be calculated taking into
      account other routing protocol parameters, such as BGP local
      preference or multi-exit discriminator (MED) attributes.

   o  Absolute topological distance, expressed in the number of
      traversed autonomous systems (AS).

   o  Absolute topological distance, expressed in the number of router
      hops (i.e., how much the TTL value of an IP packet will be
      decreased during transit).

   o  Absolute physical distance, based on knowledge of the approximate
      geo-location (e.g., continent, country) of an IP address.

   Performance-related rating criteria:

   o  The minimum achievable throughput between the resource consumer
      and the candidate resource provider, which is considered useful by
      the application (only in ALTO queries).

   o  An arbitrary upper bound for the throughput from/to the candidate
      resource provider (only in ALTO responses).  This may be, but is
      not necessarily the provisioned access bandwidth of the candidate
      resource provider.

   o  The maximum round-trip time (RTT) between resource consumer and
      the candidate resource provider, which is acceptable for the
      application for useful communication with the candidate resource
      provider (only in ALTO queries).

   o  An arbitrary lower bound for the RTT between resource consumer and
      the candidate resource provider (only in ALTO responses).  This
      may be, for example, based on measurements of the propagation
      delay in a completely unloaded network.

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   Charging-related rating criteria:

   o  Metrics representing an abstract cost, e.g., determined by
      policies that distinguish "cheap" from "expensive" IP subnet
      ranges without detailing the cost function.  The abstract metric
      "routingcost" according to [RFC7285] is an example for a metric
      for which the cost function does not have to be disclosed.

   o  Traffic volume caps, in case the Internet access of the resource
      consumer is not charged with a "flat rate".  For each candidate
      resource location, the ALTO service could indicate the amount of
      data or the bitrate that may be transferred from/to this resource
      location until a given point in time, and how much of this amount
      has already been consumed.  Furthermore, an ALTO server may have
      to indicate how excess traffic would be handled (e.g., blocked,
      throttled, or charged separately at an indicated price), e.g., by
      a new endpoint property.  This is outside the scope of this
      document.  Also, it is left for further study how several
      applications would interact if only some of them use this
      criterion.  Also left for further study is the use of such a
      criterion in resource directories that issue ALTO queries on
      behalf of other endpoints.

   All the above listed rating criteria are subject to the remarks

   The ALTO client must be aware that with high probability the actual
   performance values will differ from whatever an ALTO server exposes.
   In particular, an ALTO client must not consider a throughput
   parameter as a permission to send data at the indicated rate without
   using congestion control mechanisms.

   The discrepancies are due to various reasons, including, but not
   limited to the following facts:

   o  The ALTO service is not an admission control system.

   o  The ALTO service may not know the instantaneous congestion status
      of the network.

   o  The ALTO service may not know all link bandwidths, i.e., where the
      bottleneck really is, and there may be shared bottlenecks.

   o  The ALTO service may not have all information about the actual

   o  The ALTO service may not know whether the candidate endpoint
      itself is overloaded.

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   o  The ALTO service may not know whether the candidate endpoint
      throttles the bandwidth it devotes for the considered application.

   o  The ALTO service may not know whether the candidate endpoint will
      throttle the data it sends to the client (e.g., because of some
      fairness algorithm, such as tit-for-tat).

   Because of these inaccuracies and the lack of complete, instantaneous
   state information, which are inherent to the ALTO service, the
   application must use other mechanisms (such as passive measurements
   on actual data transmissions) to assess the currently achievable
   throughput, and it must use appropriate congestion control mechanisms
   in order to avoid a congestion collapse.  Nevertheless, the rating
   criteria may provide a useful shortcut for quickly excluding
   candidate resource providers from such probing, if it is known in
   advance that connectivity is in any case worse than what is
   considered the minimum useful value by the respective application.

   Rating criteria that should not be defined for and used by the ALTO
   service include:

   o  Performance metrics that are closely related to the instantaneous
      congestion status.  The definition of alternate approaches for
      congestion control is explicitly out of the scope of ALTO.
      Instead, other appropriate means, such as using TCP based
      transport, have to be used to avoid congestion.  In other words,
      ALTO is a service to provide network and policy information, with
      update intervals that are possibly several orders of magnitude
      slower than congestion control loops (e.g., in TCP) can react on
      changes in network congestion state.  This clear separation of
      responsibilities avoids traffic oscillations and can help for
      network stability and cost optimization.

   o  Performance metrics that raise privacy concerns.  For instance, it
      has been questioned whether an ALTO service should publicly expose
      the provisioned access bandwidth of cable / DSL customers, as this
      could enable identification of "premium customers" of an ISP.

3.3.  ALTO Focus and Scope

   The purpose of this section is ensure that administrators and users
   of ALTO services are aware of the objectives of the ALTO protocol
   design.  Using ALTO beyond this scope may limit its efficiency.
   Likewise, Map-based and Endpoint-based ALTO Services may face certain
   issues during deployment.  This section explains these limitations
   and also outlines potential solutions.

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3.3.1.  Limitations of Using ALTO Beyond Design Assumptions

   ALTO is designed as a protocol between clients integrated in
   applications and servers that provide network information and
   guidance (e.g., basic network location structure and preferences of
   network paths).  The objective is to modify network resource
   consumption patterns at application level while maintaining or
   improving application performance.  This design focus results in a
   number of characteristics of ALTO:

   o  Endpoint focus: In typical ALTO use cases, neither the consumer of
      the topology information (i.e., the ALTO client) nor the
      considered resources (e.g., files at endpoints) are part of the
      network.  The ALTO server presents an abstract network topology
      containing only information relevant to an application overlay for
      better-than-random resource provider selection among its
      endpoints.  The ALTO protocol specification [RFC7285] is not
      designed to expose network internals such as routing tables or
      configuration data that are not relevant for application-level
      resource provider selection decisions in network endpoints.

   o  Abstraction: The ALTO services such as the Network/Cost Map
      Service or the ECS provide an abstract view of the network only.
      The operator of the ALTO server has full control over the
      granularity (e.g., by defining policies how to aggregate subnets
      into PIDs) and the level-of-detail of the abstract network
      representation (e.g., by deciding what cost types to support).

   o  Multiple administrative domains: The ALTO protocol is designed for
      use cases where the ALTO server and client can be located in
      different organizations or trust domains.  ALTO assumes a loose
      coupling between server and client.  In addition, ALTO does not
      assume that an ALTO client has any a priori knowledge about the
      ALTO server and its supported features.  An ALTO server can be
      discovered automatically.

   o  Read-only: ALTO is a query/response protocol to retrieve guidance
      information.  Neither network/cost map queries nor queries to the
      endpoint cost service are designed to affect state in the network.

   If ALTO shall be deployed for use cases beyond the scope defined by
   these assumptions, the protocol design may result in limitations.

   For instance, in an Application-Based Network Operation (ABNO)
   environment the application could issue explicit service request to
   the network [RFC7491].  In this case, the application would require
   detailed knowledge about the internal network topology and the actual
   state.  A network configuration would also require a corresponding

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   security solution for authentication and authorization.  ALTO is not
   designed for operations to control, operate, and manage a network.

   Such deployments could be addressed by network management solutions,
   e.g., based on SNMP [RFC3411] or NETCONF [RFC6241] and YANG [RFC6020]
   that are typically designed to manipulate configuration state.
   Reference [RFC7491] contains a more detailed discussion of interfaces
   between components such as Element Management System (EMS), Network
   Management System (NMS), Operations Support System (OSS), Traffic
   Engineering Database (TED), Label Switched Path Database (LSP-DB),
   Path Computation Element (PCE), and other Operations, Administration,
   and Maintenance (OAM) components.

3.3.2.  Limitations of Map-based Services and Potential Solutions

   The specification of the Map Service in the ALTO protocol [RFC7285]
   is based on the concept of network maps.  A network map partitions
   the network into Provider-defined Identifiers (PIDs) that group one
   or more endpoints (e.g., subnetworks) to a single aggregate.  The
   "costs" between the various PIDs are stored in a cost map.  Map-based
   approaches such as the ALTO network and cost map service lower the
   signaling load on the server as maps have to be retrieved only if
   they change.

   One main assumption for map-based approaches is that the information
   provided in these maps is static for a long period of time.  This
   assumption is fine as long as the network operator does not change
   any parameter, e.g., routing within the network and to the upstream
   peers, IP address assignment stays stable (and thus the mapping to
   the partitions).  However, there are several cases where this
   assumption is not valid:

   1.  ISPs reallocate IP subnets from time to time.

   2.  ISPs reallocate IP subnets on short notice.

   3.  IP prefix blocks may be assigned to a router that serves a
       variety of access networks.

   4.  Network costs between IP prefixes may change depending on the
       ISP's routing and traffic engineering.

   These effects can be explained as follows:

   Case 1: ISPs may reallocate IP subnets within their infrastructure
   from time to time, partly to ensure the efficient usage of IPv4
   addresses (a scarce resource), and partly to enable efficient route
   tables within their network routers.  The frequency of these

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   "renumbering events" depend on the growth in number of subscribers
   and the availability of address space within the ISP.  As a result, a
   subscriber's household device could retain an IP address for as short
   as a few minutes, or for months at a time or even longer.

   It has been suggested that ISPs providing ALTO services could sub-
   divide their subscribers' devices into different IP subnets (or
   certain IP address ranges) based on the purchased service tier, as
   well as based on the location in the network topology.  The problem
   is that this sub-allocation of IP subnets tends to decrease the
   efficiency of IP address allocation, in particular for IPv4.  A
   growing ISP that needs to maintain high efficiency of IP address
   utilization may be reluctant to jeopardize their future acquisition
   of IP address space.

   However, this is not an issue for map-based approaches if changes are
   applied in the order of days.

   Case 2: ISPs can use techniques that allow the reallocation of IP
   prefixes on very short notice, i.e., within minutes.  An IP prefix
   that has no IP address assignment to a host anymore can be
   reallocated to areas where there is currently a high demand for IP

   Case 3: In residential access networks (e.g., DSL, cable), IP
   prefixes are assigned to broadband gateways, which are the first IP-
   hop in the access-network between the Customer Premises Equipment
   (CPE) and the Internet.  The access-network between CPE and broadband
   gateway (called aggregation network) can have varying characteristics
   (and thus associated costs), but still using the same IP prefix.  For
   instance one IP address IP1 out of a given CIDR prefix can be
   assigned to a VDSL access line (e.g., 2 MBit/s uplink) while another
   IP address IP2 within the same given CIDR prefix is assigned to a
   slow ADSL line (e.g., 128 kbit/s uplink).  These IP addresses may be
   assigned on a first come first served basis, i.e., a single IP
   address out of the same CIDR prefix can change its associated costs
   quite fast.  This may not be an issue with respect to the used
   upstream provider (thus the cross ISP traffic) but depending on the
   capacity of the aggregation-network this may raise to an issue.

   Case 4: The routing and traffic engineering inside an ISP network, as
   well as the peering with other autonomous systems, can change
   dynamically and affect the information exposed by an ALTO server.  As
   a result, cost maps and possibly also network maps can change.

   One solution to deal with map changes is to use incremental ALTO
   updates [I-D.ietf-alto-incr-update-sse].

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3.3.3.  Limitations of Non-Map-based Services and Potential Solutions

   The specification of the ALTO protocol [RFC7285] also includes the
   Endpoint Cost Service (ECS) mechanism.  ALTO clients can ask the ALTO
   server for guidance for specific IP addresses, thereby avoiding the
   need of processing maps.  This can mitigate some of the problems
   mentioned in the previous section.

   However frequent requests, particularly with long lists of IP
   addresses, may overload the ALTO server.  The server has to rank each
   received IP address, which causes load at the server.  This may be
   amplified when not only a single ALTO client is asking for guidance,
   but a large number of them.  The results of the ECS are also more
   difficult to cache than ALTO maps.  Therefore, the ALTO client may
   have to await the server response before starting a communication,
   which results in an additional delay.

   Caching of IP addresses at the ALTO client or the usage of the H12
   approach [I-D.kiesel-alto-h12] in conjunction with caching may lower
   the query load on the ALTO server.

   When ALTO server receives an ECS request, it may not have the most
   appropriate topology information in order to accurately determine the
   ranking.  [RFC7285] generally assumes that a server can always offer
   some guidance.  In such a case the ALTO server could adopt one of the
   following strategies:

   o  Reply with available information (best effort).

   o  Query another ALTO server presumed to have better topology
      information and return that response (cascaded servers).

   o  Redirect the request to another ALTO server presumed to have
      better topology information (redirection).

   The protocol mechanisms and decision processes that would be used to
   determine if redirection is necessary and which mode to use is out of
   the scope of this document, since protocol extensions could be

3.4.  Monitoring ALTO

3.4.1.  Impact and Observation on Network Operation

   ALTO presents a new opportunity for managing network traffic by
   providing additional information to clients.  In particular, the
   deployment of an ALTO server may shift network traffic patterns, and
   the potential impact to network operation can be large.  An ISP

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   providing ALTO may want to assess the benefits of ALTO as part of the
   management and operations (cf. [RFC7285]).  For instance, the ISP
   might be interested in understanding whether the provided ALTO maps
   are effective, and in order to decide whether an adjustment of the
   ALTO configuration would be useful.  Such insight can be obtained
   from a monitoring infrastructure.  An ISP offering ALTO could
   consider the impact on (or integration with) traffic engineering and
   the deployment of a monitoring service to observe the effects of ALTO
   operations.  The measurement of impacts can be challenging because
   ALTO-enabled applications may not provide related information back to
   the ALTO service provider.

   To construct an effective monitoring infrastructure, the ALTO service
   provider should decide how to monitor the performance of ALTO and
   identify and deploy data sources to collect data to compute the
   performance metrics.  In certain trusted deployment environments, it
   may be possible to collect information directly from ALTO clients.
   It may also be possible to vary or selectively disable ALTO guidance
   for a portion of ALTO clients either by time, geographical region, or
   some other criteria to compare the network traffic characteristics
   with and without ALTO.  Monitoring an ALTO service could also be
   realized by third parties.  In this case, insight into ALTO data may
   require a trust relationship between the monitoring system operator
   and the network service provider offering an ALTO service.

   The required monitoring depends on the network infrastructure and the
   use of ALTO, and an exhaustive description is outside the scope of
   this document.

3.4.2.  Measurement of the Impact

   ALTO realizes an interface between the network and applications.
   This implies that an effective monitoring infrastructure may have to
   deal with both network and application performance metrics.  This
   document does not comprehensively list all performance metrics that
   could be relevant, nor does it formally specify metrics.

   The impact of ALTO can be classified regarding a number of different

   o  Total amount and distribution of traffic: ALTO enables ISPs to
      influence and localize traffic of applications that use the ALTO
      service.  An ISP may therefore be interested in analyzing the
      impact on the traffic, i.e., whether network traffic patterns are
      shifted.  For instance, if ALTO shall be used to reduce the inter-
      domain P2P traffic, it makes sense to evaluate the total amount of
      inter-domain traffic of an ISP.  Then, one possibility is to study
      how the introduction of ALTO reduces the total inter-domain

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      traffic (inbound and/our outbound).  If the ISPs intention is to
      localize the traffic inside his network, the network-internal
      traffic distribution will be of interest.  Effectiveness of
      localization can be quantified in different ways, e.g., by the
      load on core routers and backbone links, or by considering more
      advanced effects, such as the average number of hops that traffic
      traverses inside a domain.

   o  Application performance: The objective of ALTO is improve
      application performance.  ALTO can be used by very different types
      applications, with different communication characteristics and
      requirements.  For instance, if ALTO guidance achieves traffic
      localization, one would expect that applications achieve a higher
      throughput and/or smaller delays to retrieve data.  If
      application-specific performance characteristics (e.g., video or
      audio quality) can be monitored, such metrics related to user
      experience could also help to analyze the benefit of an ALTO
      deployment.  If available, selected statistics from the TCP/IP
      stack in hosts could be leveraged, too.

   Of potential interest can also be the share of applications or
   customers that actually use an offered ALTO service, i.e., the
   adoption of the service.

   Monitoring statistics can be aggregated, averaged, and normalized in
   different ways.  This document does not mandate specific ways how to
   calculate metrics.

3.4.3.  System and Service Performance

   A number of interesting parameters can be measured at the ALTO
   server.  [RFC7285] suggests certain ALTO-specific metrics to be

   o  Requests and responses for each service listed in a Information
      Directory (total counts and size in bytes).

   o  CPU and memory utilization

   o  ALTO map updates

   o  Number of PIDs

   o  ALTO map sizes (in-memory size, encoded size, number of entries)

   This data characterizes the workload, the system performance as well
   as the map data.  Obviously, such data will depend on the
   implementation and the actual deployment of the ALTO service.

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   Logging is also recommended in [RFC7285].

3.4.4.  Monitoring Infrastructures

   Understanding the impact of ALTO may require interaction between
   different systems, operating at different layers.  Some information
   discussed in the preceding sections is only visible to an ISP, while
   application-level performance can hardly be measured inside the
   network.  It is possible that not all information of potential
   interest can directly be measured, either because no corresponding
   monitoring infrastructure or measurement method exists, or because it
   is not easily accessible.

   One way to quantify the benefit of deploying ALTO is to measure
   before and after enabling the ALTO service.  In addition to passive
   monitoring, some data could also be obtained by active measurements,
   but due to the resulting overhead, the latter should be used with
   care.  Yet, in all monitoring activities an ALTO service provider has
   to take into account that ALTO clients are not bound to ALTO server
   guidance as ALTO is only one source of information, and any
   measurement result may thus be biased.

   Potential sources for monitoring the use of ALTO include:

   o  Network monitoring and performance management systems: Many ISPs
      deploy systems to monitor the network traffic, which may have
      insight into traffic volumes, network topology, bandwidth
      information inside the management area.  Data can be obtained by
      SNMP, NETCONF, IP Flow Information Export (IPFIX), syslog, etc.
      On-demand OAM tests (such as Ping or BDF) could also be used.

   o  Applications/clients: Relevant data could be obtained by
      instrumentation of applications.

   o  ALTO server: If available, log files or other statistics data
      could be analyzed.

   o  Other application entities: In several use cases, there are other
      application entities that could provide data as well.  For
      instance, there may be centralized log servers that collect data.

   In many ALTO use cases some data sources are located within an ISP
   network while some other data is gathered at application level.
   Correlation of data could require a collaboration agreement between
   the ISP and an application owner, including agreements of data
   interchange formats, methods of delivery, etc.  In practice, such a
   collaboration may not be possible in all use cases of ALTO, because
   the monitoring data can be sensitive, and because the interacting

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   entities may have different priorities.  Details of how to build an
   over-arching monitoring system for evaluating the benefits of ALTO
   are outside the scope of this memo.

3.5.  Abstract Map Examples for Different Types of ISPs

3.5.1.  Small ISP with Single Internet Uplink

   The ALTO protocol does not mandate how to determine costs between
   endpoints and/or determine map data.  In complex usage scenarios this
   can be a non-trivial problem.  In order to show the basic principle,
   this and the following sections explain for different deployment
   scenarios how ALTO maps could be structured.

   For a small ISP, the inter-domain traffic optimizing problem is how
   to decrease the traffic exchanged with other ISPs, because of high
   settlement costs.  By using the ALTO service to optimize traffic, a
   small ISP can define two "optimization areas": one is its own
   network; the other one consists of all other network destinations.
   The cost map can be defined as follows: the cost of a link between
   clients of the inner ISP's network is lower than between clients of
   the outer ISP's network and clients of inner ISP's network.  As a
   result, a host with an ALTO client inside the network of this ISP
   will prefer retrieving data from hosts connected to the same ISP.

   An example is given in Figure 9.  It is assumed that ISP A is a small
   ISP only having one access network.  As operator of the ALTO service,
   ISP A can define its network to be one optimization area, named as
   PID1, and define other networks to be the other optimization area,
   named as PID2.  C1 is denoted as the cost inside the network of ISP
   A. C2 is denoted as the cost from PID2 to PID1, and C3 from PID1 to
   PID2.  For the sake of simplicity, in the following C2=C3 is assumed.
   In order to keep traffic local inside ISP A, it makes sense to define

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          ////           \\\\
        //                   \\
      //                       \\                  /-----------\
     | +---------+               |             ////             \\\\
     | | ALTO    |  ISP A        |    C2      |    Other Networks   |
    |  | Service |  PID 1         <-----------     PID 2
     | +---------+  C1           |----------->|                     |
     |                           |  C3 (=C2)   \\\\             ////
      \\                       //                  \-----------/
        \\                   //
          \\\\           ////

             Figure 9: Example ALTO deployment for a small ISP

   A simplified extract of the corresponding ALTO network and cost maps
   is listed in Figure 10 and Figure 11, assuming that the network of
   ISP A has the IPv4 address ranges and,
   as well as the IPv6 address range 2001:db8:100::/48.  In this
   example, the cost values C1 and C2 can be set to any number C1<C2.

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      HTTP/1.1 200 OK
      Content-Type: application/alto-networkmap+json

        "network-map" : {
          "PID1" : {
            "ipv4" : [
            "ipv6" : [
          "PID2" : {
            "ipv4" : [
            "ipv6" : [

                    Figure 10: Example ALTO network map

      HTTP/1.1 200 OK
      Content-Type: application/alto-costmap+json

          "cost-type" : {"cost-mode"  : "numerical",
                         "cost-metric": "routingcost"
        "cost-map" : {
          "PID1": { "PID1": C1,  "PID2": C2 },
          "PID2": { "PID1": C2,  "PID2": 0 },

                     Figure 11: Example ALTO cost map

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3.5.2.  ISP with Several Fixed Access Networks

   This example discusses a P2P application traffic optimization use
   case for a larger ISP with a fixed network comprising several access
   networks and a core network.  The traffic optimizing objectives
   include (1) using the backbone network efficiently, (2) adjusting the
   traffic balance in different access networks according to traffic
   conditions and management policies, and (3) achieving a reduction of
   settlement costs with other ISPs.

   Such a large ISP deploying an ALTO service may want to optimize its
   traffic according to the network topology of its access networks.
   For example, each access network could be defined to be one
   optimization area, i.e., traffic should be kept local withing that
   area if possible.  This can be achieved by mapping each area to a
   PID.  Then the costs between those access networks can be defined
   according to a corresponding traffic optimizing requirement by this
   ISP.  One example setup is further described below and also shown in
   Figure 12.

   In this example, ISP A has one backbone network and three access
   networks, named as AN A, AN B, and AN C. A P2P application is used in
   this example.  For a reasonable application-level traffic
   optimization, the first requirement could be a decrease of the P2P
   traffic on the backbone network inside the Autonomous System of ISP A
   and the second requirement could be a decrease of the P2P traffic to
   other ISPs, i.e., other Autonomous Systems.  The second requirement
   can be assumed to have priority over the first one.  Also, we assume
   that the settlement rate with ISP B is lower than with other ISPs.
   ISP A can deploy an ALTO service to meet these traffic distribution
   requirements.  In the following, we will give an example of an ALTO
   setting and configuration according to these requirements.

   In the network of ISP A, the operator of the ALTO server can define
   each access network to be one optimization area, and assign one PID
   to each access network, such as PID 1, PID 2, and PID 3.  Because of
   different peerings with different outer ISPs, one can define ISP B to
   be one additional optimization area and assign PID 4 to it.  All
   other networks can be added to a PID to be one further optimization
   area (PID 5).

   In the setup, costs (C1, C2, C3, C4, C5, C6, C7, C8) can be assigned
   as shown in Figure 12.  Cost C1 is denoted as the link cost in inner
   AN A (PID 1), and C2 and C3 are defined accordingly.  C4 is denoted
   as the link cost from PID 1 to PID 2, and C5 is the corresponding
   cost from PID 3, which is assumed to have a similar value.  C6 is the
   cost between PID 1 and PID 3.  For simplicity, this scenario assumes
   symmetrical costs between the AN this example.  C7 is denoted as the

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   link cost from the ISP B to ISP A. C8 is the link cost from other
   networks to ISP A.

   According to previous discussion of the first requirement and the
   second requirement, the relationship of these costs will be defined
   as: (C1, C2, C3) < (C4, C5, C6) < (C7) < (C8)

    +------------------------------------+         +----------------+
    | ISP A   +---------------+          |         |                |
    |         |    Backbone   |          |   C7    |      ISP B     |
    |     +---+    Network    +----+     |<--------+      PID 4     |
    |     |   +-------+-------+    |     |         |                |
    |     |           |            |     |         |                |
    |     |           |            |     |         +----------------+
    | +---+--+     +--+---+     +--+---+ |
    | |AN A  |  C4 |AN B  |  C5 |AN C  | |
    | |PID 1 +<--->|PID 2 |<--->+PID 3 | |
    | |C1    |     |C2    |     |C3    | |         +----------------+
    | +---+--+     +------+     +--+---+ |         |                |
    |     ^                        ^     |   C8    | Other Networks |
    |     |                        |     |<--------+ PID 5          |
    |     +------------------------+     |         |                |
    |                  C6                |         |                |
    +------------------------------------+         +----------------+

    Figure 12: ALTO deployment in large ISPs with layered fixed network

3.5.3.  ISP with Fixed and Mobile Network

   An ISP with both mobile network and fixed network may focus on
   optimizing the mobile traffic by keeping traffic in the fixed network
   as much as possible, because wireless bandwidth is a scarce resource
   and traffic is costly in mobile network.  In such a case, the main
   requirement of traffic optimization could be decreasing the usage of
   radio resources in the mobile network.  An ALTO service can be
   deployed to meet these needs.

   Figure 13 shows an example: ISP A operates one mobile network, which
   is connected to a backbone network.  The ISP also runs two fixed
   access networks AN A and AN B, which are also connected to the
   backbone network.  In this network structure, the mobile network can
   be defined as one optimization area, and PID 1 can be assigned to it.
   Access networks AN A and B can also be defined as optimization areas,
   and PID 2 and PID 3 can be assigned, respectively.  The cost values
   are then defined as shown in Figure 13.

   To decrease the usage of wireless link, the relationship of these

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   costs can be defined as follows:

   From view of mobile network: C4 < C1 and C4 = C8.  This means that
   clients in mobile network requiring data resources from other clients
   will prefer clients in AN A or B to clients in the mobile network.
   This policy can decrease the usage of wireless link and power
   consumption in terminals.

   From view of AN A: C2 < C6, C5 = maximum cost.  This means that
   clients in other optimization area will avoid retrieving data from
   the mobile network.

   From view of AN B: Analog to the view of AN A, C3 < C8 and C9 =
   maximum cost.

   |                                                                 |
   |  ISP A                 +-------------+                          |
   |               +--------+   ALTO      +---------+                |
   |               |        |   Service   |         |                |
   |               |        +------+------+         |                |
   |               |               |                |                |
   |               |               |                |                |
   |               |               |                |                |
   |       +-------+-------+       | C6    +--------+------+         |
   |       |     AN A      |<--------------|      AN B     |         |
   |       |     PID 2     |   C7  |       |      PID 3    |         |
   |       |     C2        |-------------->|      C3       |         |
   |       +---------------+       |       +---------------+         |
   |             ^    |            |              |     ^            |
   |             |    |            |              |     |            |
   |             |    | C4         |           C8 |     |            |
   |          C5 |    |            |              |     | C9         |
   |             |    |   +--------+---------+    |     |            |
   |             |    +-->|  Mobile Network  |<---+     |            |
   |             |        |  PID 1           |          |            |
   |             +------- |  C1              |----------+            |
   |                      +------------------+                       |

          Figure 13: ALTO deployment in ISPs with mobile network

   These examples show that for ALTO in particular the relationships
   between different costs matter; the operator of the server has
   several degrees of freedom how to set the absolute values.

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3.6.  Comprehensive Example for Map Calculation

   In addition to the previous, abstract examples, this section presents
   a more detailed scenario with a realistic IGP and BGP routing
   protocol configuration.  This example was first described in

3.6.1.  Example Network

   Figure 14 depicts a network which is used to explain the steps
   carried out in the course of this example.  The network consists of
   nine routers (R1 to R9).  Two of them are border routers (R1 + R8)
   connected to neighbored networks (AS 2 to AS 4).  Furthermore, AS 4
   is not directly connected to the local network, but has AS 3 as
   transit network.  The links between the routers are point-to-point
   connections.  These connections also form the core network with the
   2001:db8:1:0::/56 prefix.  This prefix is large enough to provide
   addresses for all router interconnections.  In addition to the core
   network, the local network also has five client networks attached to
   five different routers (R2, R5, R6, R7 and R9).  Each client network
   has a /56 prefix with 2001:db8:1:x00:: (x = [1..5]) as network

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   +-------------------+    +-----+    +-----+    +-------------------+
   |2001:db8:1:200::/56+----+ R6  |    | R7  +----+2001:db8:1:300::/56|
   +-------------------+    +--+--+    +--+--+    +-------------------+
                               |          |
   +---------------+           |          |
   |      AS 2     |           |          |
   |2001:db8:2::/48|           | 10       | 10
   +------------+--+           |          |
                |              |          |
                |              |          |
             +--+--+   15   +--+--+    +--+--+    +-------------------+
             | R1  +--------+ R3  +----+ R5  |----+2001:db8:1:400::/56|
             +--+--+        +--+--+ 5  +--+--+    +-------------------+
                |   \      /   |          |
                |    \    / 15 |          |
                |     \  /     |          |           +---------------+
                |      \/      |          |           |      AS 4     |
                | 20   /\      | 5        | 10        |2001:db8:4::/48|
                |     /  \     |          |           +-------+-------+
                |    /    \ 20 |          |                   |
                |   /      \   |          |                   |
             +--+--+        +--+--+    +--+--+        +-------+-------+
             | R2  |        | R4  |    | R8  +--------+      AS 3     |
             +--+--+        +--+--+    +--+--+        |2001:db8:3::/48|
                |              |          |           +---------------+
                |              |          | 10
                |              | 20       |
   +------------+------+       |       +--+--+    +-------------------+
   |2001:db8:1:100::/56|       +-------+ R9  +----+2001:db8:1:500::/56|
   +-------------------+               +-----+    +-------------------+

                        Figure 14: Example network

   The example network utilizes two different routing protocols, one for
   IGP and another for EGP routing.  The used IGP is a link-state
   protocol such as IS-IS.  The applied link weights are annotated in
   the graph and additionally shown in Figure 15.  All links are
   bidirectional and their weights are symmetric.  To obtain the
   topology and routing information from the network, the topology data
   source must be connected directly to one of the routers (R1...R9).
   Furthermore, the topology data source must be enabled to communicate
   with the router and vice versa.

   The Border Gateway Protocol (BGP) is used in this scenario to route
   between autonomous systems (AS).  External BGP is running on the two
   border routers R1 and R8.  Furthermore, internal BGP is used to
   propagate external as well as internal prefixes within the network
   boundaries; it is running on every router with an attached client

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   network (R2, R5, R6, R7 and R9).  Since no route reflector is present
   it is necessary to fetch routes from each BGP router separately.

              R1   R2   R3   R4   R5   R6   R7   R8   R9
          R1   0   15   15   20    -    -    -    -    -
          R2  15    0   20    -    -    -    -    -    -
          R3  15   20    0    5    5   10    -    -    -
          R4  20    -    5    0    5    -    -    -   20
          R5   -    -    5    5    0    -   10   10    -
          R6   -    -   10    -    -    0    -    -    -
          R7   -    -    -    -   10    -    0    -    -
          R8   -    -    -    -   10    -    -    0   10
          R9   -    -    -   20    -    -    -   10    0

                  Figure 15: Example network link weights

   For monitoring purposes it is possible to enable e.g.  SNMP or
   NETCONF on the routers within the network.  This way an ALTO server
   may obtain several additional information about the state of the
   network.  For example, utilization, latency, and bandwidth
   information could be retrieved periodically from the network
   components to get and keep an up-to-date view on the network

   In the following, it is assumed that the listed attributes are
   collected from the network:

   o  IS-IS: topology, link weights

   o  BGP: prefixes, AS numbers, AS distances, or other BGP metrics

   o  SNMP: latency, utilization, bandwidth

3.6.2.  Potential Input Data Processing and Storage

   Due to the variety of data source available in a network it may be
   necessary to aggregate the information and define a suitable data
   model that can hold the information efficiently and easily
   accessible.  One potential model is an annotated directed graph that
   represents the topology.  The attributes can be annotated at the
   corresponding positions in the graph.  In the following it is shown
   how such a topology graph could describe the example topology.

   In the topology graph, a node represents a router in the network,
   while the edges stand for the links that connect the routers.  Both
   routers and links have a set of attributes that store information
   gathered from the network.

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   Each router could be associated with a basic set of information, such

   o  ID: Unique ID within the network to identify the router.

   o  Neighbor IDs: List of directly connected routers.

   o  Endpoints: List of connected endpoints.  The endpoints may also
      have further attributes themselves depending on the network and
      address type.  Such potential attributes are costs for reaching
      the endpoint from the router, AS numbers, or AS distances.

   In addition to the basic set many more attributes may be assigned to
   router nodes.  This mainly depends on the utilized data sources.
   Examples for such additional attributes are geographic location, host
   name and/or interface types, just to name a few.

   The example network shown in Figure 14 represents such an internal
   network graph where the routers R1 to R9 represent the nodes and the
   connections between them are the links.  For instance, R2 has one
   directly attached IPv6 endpoint that belongs to its own AS, as shown
   in Figure 16.

      ID:  2

      Neighbor IDs:  1,3 (R1, R3)


         Endpoint:  2001:db8:1:100::/56

         Weight:  10 (e.g., the default IGP metric value)

         ASNumber:  1 (our own AS)

         ASDistance:  0

      Host Name:  R2

                       Figure 16: Example router R2

   Router R8 has two attached IPv6 endpoints, as explained in Figure 17.
   The first one belongs to a directly neighbored AS with AS number 3.
   The AS distance from our network to AS3 is 1.  The second endpoint
   belongs to an AS (AS4) that is no direct neighbor but directly
   connected to AS3.  To reach endpoints in AS4 it is necessary to cross
   AS3, which increases the AS distance by one.

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      ID:  8

      Neighbor IDs:  5,9 (R5, R9)


         Endpoint:  2001:db8:3::/48

         Weight:  100

         ASNumber:  3

         ASDistance:  1

         Endpoint:  2001:db8:4::/48

         Weight:  200

         ASNumber:  4

         ASDistance:  2

      Host Name:  R8

                       Figure 17: Example router R8

   A potential set of attributes for a link is described in the
   following list:

   o  Source ID: ID of the source router of the link.

   o  Destination ID: ID of the destination router of the link.

   o  Weight: The cost to cross the link, e.g., defined by the used IGP.

   Additional attributes that provide technical details and state
   information can be assigned to links as well.  The availability of
   such additional attributes depends on the utilized data sources.
   Such attributes can be characteristics like maximum bandwidth,
   utilization, or latency on the link as well as the link type.

   In the example, the link attributes are equal for all links and only
   their values differ.  It is assumed that the attributes utilization,
   bandwidth, and latency are collected e.g. via SNMP or NETCONF.  In
   the topology of Figure 14 the links between R1 and R2 would then have
   the following link attributes explained in Figure 18:

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      Source ID:  1

      Destination ID:  2

      Weight:  15

      Bandwidth:  10Gbit/s

      Utilization:  0.1

      Latency:  2ms


      Source ID:  2

      Destination ID:  1

      Weight:  15

      Bandwidth:  10Gbit/s

      Utilization:  0.55

      Latency:  5ms

                        Figure 18: Link attributes

   It has to be emphasized that values for utilization and latency can
   be very volatile.

3.6.3.  Calculation of Network Map from the Input Data

   The goal of the ALTO map calculation process is to get from the graph
   representation of the network to a coarser-grained and abstract
   matrix representation.  The first step is to generate the network
   map.  Only after the network map has been generated it is possible to
   compute the cost map, since it relies on the network map.

   To generate an ALTO network map a grouping function is required.  A
   grouping function processes information from the network graph to
   group endpoints into PIDs.  The way of grouping is manifold and
   algorithms can utilize any information provided by the network graph
   to perform the grouping.  The functions may omit certain endpoints in
   order to simplify the map or in order to hide details about the
   network that are not intended to be published in the resulting ALTO

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   network map.

   For IP endpoints, which are either an IP (version 4 or version 6)
   address or prefix, [RFC7285] requires the use of longest-prefix
   matching algorithm to map IPs to PIDs.  This requirement results in
   the constraint that every IP must be mapped to a PID and that the
   same prefix or address is not mapped to more than one PID.  To meet
   the first constraint every calculated map must provide a default PID
   that contains the prefixes for IPv4 and ::/0 for IPv6.
   Both prefixes cover their entire address space and if no other PID
   matches an IP endpoint the default PID will.  The second constraint
   must be met by the grouping function that assigns endpoints to PIDs.
   In case of collision the grouping function must decide to which PID
   an endpoint is assigned.  These or other constraints may apply to
   other endpoint types depending on the used matching algorithm.

   A simple example for such grouping is to compose PIDs by host names.
   For instance, each router's host name is selected as the name for a
   PID and the attached endpoints are the member endpoints of the
   corresponding PID.  Additionally, backbone prefixes should not appear
   in the map so they are filtered out.  The following table in
   Figure 19 shows the resulting ALTO network map, using the network in
   Figure 14 as example:

          PID  |  Endpoints
           R1  |  2001:db8:2::/48
           R2  |  2001:db8:1:100::/56
           R5  |  2001:db8:1:400::/56
           R6  |  2001:db8:1:200::/56
           R7  |  2001:db8:1:300::/56
           R8  |  2001:db8:3::/48, 2001:db8:4::/48
           R9  |  2001:db8:1:500::/56
       default |, ::/0

                    Figure 19: Example ALTO network map

   Since router R3 and R4 have no endpoints assigned they are not
   represented in the network map.  Furthermore, as previously
   mentioned, the "default" PID was added to represent all endpoints
   that are not part of the example network.

3.6.4.  Calculation of Cost Map

   After successfully creating the network map, the typical next step is
   to calculate the costs between the generated PIDs, which form the
   cost map.  Those costs are calculated by cost functions.  A cost
   function may calculate unidirectional values, which means it is

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   necessary to compute the costs from every PID to every PID.  In
   general, it is possible to use all available information in the
   network graph to compute the costs.  In case a PID contains more than
   one IP address or prefix, the cost function may first calculate a set
   of cost values for each source/destination IP pair.  In that case, a
   tie-breaker function is required to decide the resulting cost value
   as [RFC7285] allows one cost value only between 2 PIDs.  Such tie-
   breaker can be a simple function such as minimum, maximum, or average

   No matter what metric the cost function is using, the path from
   source to destination is usually defined by the path with minimum
   weight.  When the link weight is represented by an additive metric,
   the path weight is the sum of link weights of all traversed links.
   The path may be determined for instance with the Bellman-Ford or
   Dijkstra algorithm.  The latter progressively builds the shortest
   path in terms of cumulated link lengths.  In our example, the link
   lengths are link weights with values illustrated in Figure 15.
   Hence, the cost function generally extracts the optimal path with
   respect to a chosen metric, such as the IGP link weight.  It is also
   possible that more than one path with the same minimum weight exist,
   which means it is not entirely clear which path is going to be
   selected by the network.  Hence, a tie-breaker similar to the one
   used to resolve costs for PIDs with multiple endpoints is necessary.

   An important note is that [RFC7285] does not require cost maps to
   provide costs for every PID pair, so if no path cost can be
   calculated for a certain pair, the corresponding field in the cost
   map is left out.  Administrators may also not want to provide cost
   values for some PID pairs due to various reasons.  Such pairs may be
   defined before the cost calculation is performed.

   Based on the network map example shown in the previous section it is
   possible to calculate the cost maps.  Figure 20 provides an example
   where the selected metric for the cost map is the minimum number of
   hops necessary to get from the endpoints in the source PID to
   endpoints in the destination PID.  Our chosen tie-breaker selects the
   minimum hop count when more than one value is returned by the cost

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         PID  | default | R1  | R2  | R5  | R6  | R7  | R8  | R9  |
      default |    x    |  x  |  x  |  x  |  x  |  x  |  x  |  x  |
         R1   |    x    |  0  |  2  |  3  |  3  |  4  |  4  |  3  |
         R2   |    x    |  2  |  0  |  3  |  3  |  4  |  4  |  4  |
         R5   |    x    |  3  |  3  |  0  |  3  |  2  |  2  |  3  |
         R6   |    x    |  3  |  3  |  3  |  0  |  4  |  4  |  4  |
         R7   |    x    |  4  |  4  |  2  |  4  |  0  |  3  |  4  |
         R8   |    x    |  4  |  4  |  2  |  4  |  3  |  0  |  2  |
         R9   |    x    |  3  |  4  |  3  |  4  |  4  |  2  |  0  |

                 Figure 20: Example ALTO hopcount cost map

   It should be mentioned that R1->R9 has several paths with equal path
   weights.  The paths R1->R3->R5->R8->R9, R1->R3->R4->R9 and R1->R4->R9
   all have a path weight of 40.  Due to the minimum hopcount value tie-
   breaker, 3 hops is chosen as value for the path R1->R4->R9.
   Furthermore, since the "default" PID is, in a sense, a virtual PID
   with no endpoints that are part of the example network, no cost
   values are calculated for other PIDs from or towards it.

3.7.  Deployment Experiences

   There are multiple interoperable implementations of the ALTO
   protocol.  Some experiences in implementating and using ALTO for
   large-scale networks have been documented in
   [I-D.seidel-alto-map-calculation] and are here summarized:

   o  Data collection: Retrieving topology information typically
      requires implementing several protocols other than ALTO for data
      collection.  For such other protocols, ALTO deployments faced
      protocol behaviors that were different to what would be expected
      from the specification of the corresponding protocol.  This
      includes behavior caused by older versions of the protocol
      specification, a lax interpretation on the remote side or simply
      incompatibility with the corresponding standard.  This sort of
      problems in collecting data can make an ALTO deployment more
      complicated, even if it is unrelated to ALTO protocol itself.

   o  Data processing: Processing network information can be very
      complex and quite resource-demanding.  Gathering information from
      an autonomous system connected to Internet may imply that a server
      must store and process hundreds of thousands of prefixes, several
      hundreds of megabytes of IPFIX/Netflow information per minute, and
      information from hundreds of routers and attributes of thousands
      of links.  A lot of disk memory, RAM, and CPU cycles as well as
      efficient algorithms are required to process the information.
      Operators of an ALTO server have to be aware that significant

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      compute resources are not only required for the ALTO server, but
      also for the corresponding data collection.

   o  Network map calculation: Large IP based networks consist of
      hundreds of thousands of prefixes, which have to be mapped to PIDs
      in the process of network map calculation.  As a result, network
      maps get very large (up to tens of megabytes).  However, depending
      on the design of the network and the chosen grouping function the
      calculated network maps contains redundancy that can be removed.
      There are at least two ways to reduce the size by removing
      redundancy.  First, adjacent IP prefixes can be merged.  When a
      PID has two adjacent prefix entries it can merge them together to
      one larger prefix.  It is mandatory that both prefixes are in the
      same PID.  However, it cannot be ruled out that the large prefix
      is assigned to another PID.  This must be checked and it is up to
      the grouping function whether it merges the prefixes and removes
      the larger prefix from the other PID or not.  A simple example,
      when a PID comprises the prefixes 2001:db8:0:0::/64 and 2001:db8:
      0:1::/64 it can easily merge them to 2001:db8:0:0::/63.  Second, a
      prefix and its next-longer-prefix match may be in the same PID.
      In this case, the smaller prefix can simply be removed since it is
      redundant for obvious reasons.  A simple example, a PID comprises
      the prefixes 2001:db8:0:0::/62 and 2001:db8:0:1::/64 and the /62
      is the next-longer prefix match of the /64, the /64 prefix can
      simply be removed.  In contrast, if another PID contains the 2001:
      db8:0:0::/63 prefix, the entry 2001:db8:0:1::/64 cannot be removed
      since the next-longer prefix is not in the same PID anymore.
      Operators of an ALTO server thus have to analyze whether their
      address assignment schemes allows such tuning.

   o  Cost map calculation: One known implementation challenge with cost
      map calculations is the vast amount of CPU cycles that may be
      required to calculate the costs in large networks.  This is
      particular problematic if costs are calculated between the
      endpoints of each source-destination PID pair.  Very often several
      to many endpoints of a PID are attached to the same node, so the
      same path cost is calculated several times.  This is clearly
      inefficient.  A remedy could be more sophisticated algorithms,
      such as looking up the routers the endpoints of each PID are
      connected to in our network graph and calculated cost map based on
      the costs between the routers.  When deploying and configuring
      ALTO servers, administrators should consider the impact of huge
      cost maps and possibly ensure that map sizes do not get too large.

   In addition, further deployment experiences have been documented.
   One real example is described in greater detail in reference

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   Also, experiments have been conducted with ALTO-like deployments in
   Internet Service Provider (ISP) networks.  For instance, NTT
   performed tests with their HINT server implementation and dummy nodes
   to gain insight on how an ALTO-like service can influence peer-to-
   peer systems [RFC6875].  The results of an early experiment conducted
   in the Comcast network are documented in [RFC5632].

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4.  Using ALTO for P2P Traffic Optimization

4.1.  Overview

4.1.1.  Usage Scenario

   Originally, peer-to-peer (P2P) applications were the main driver for
   the development of ALTO.  In this use case it is assumed that one
   party (usually the operator of a "managed" IP network domain) will
   disclose information about the network through ALTO.  The application
   overlay will query this information and optimize its behavior in
   order to improve performance or Quality of Experience in the
   application while reducing the utilization of the underlying network
   infrastructure.  The resulting win-win situation is assumed to be the
   incentive for both parties to provide or consume the ALTO
   information, respectively.

   P2P systems can be built with or without use of a centralized
   resource directory ("tracker").  The scope of this section is the
   interaction of P2P applications with the ALTO service.  In this
   scenario, the resource consumer ("peer") asks the resource directory
   for a list of candidates that can provide the desired resource.
   There are different options for how ALTO can be deployed in such use
   cases with a centralized resource directory.

   For efficiency reasons (i.e., message size), only a subset of all
   resource providers known to the resource directory will be returned
   to the resource consumer.  Some or all of these resource providers,
   plus further resource providers learned by other means such as direct
   communication between peers, will be contacted by the resource
   consumer for accessing the resource.  The purpose of ALTO is giving
   guidance on this peer selection, which should yield better-than-
   random results.  The tracker response as well as the ALTO guidance
   are most beneficial in the initial phase after the resource consumer
   has decided to access a resource, as long as only few resource
   providers are known.  Later, when the resource consumer has already
   exchanged some data with other peers and measured the transmission
   speed, the relative importance of ALTO may dwindle.

4.1.2.  Applicability of ALTO

   A tracker-based P2P application can leverage ALTO in different ways.
   In the following, the different alternatives and their pros and cons
   are discussed.

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                            ,-------.         +-----------+
          ,---.          ,-'         ========>|   Peer 1  |********
       ,-'     `-.      /     ISP 1  V  \     |ALTO Client|       *
      /           \    / +-------------+ \    +-----------+       *
     /    ISP X    \   | + ALTO Server | |    +-----------+       *
    /               \  \ +-------------+<====>|   Peer 2  |       *
   ;   +---------+   :  \               /     |ALTO Client|****** *
   |   | Global  |   |   `-.         ,-'      +-----------+     * *
   |   | Tracker |   |      `-------'                           * *
   |   +---------+   |      ,-------.         +-----------+     * *
   :        *        ;   ,-'         ========>|   Peer 3  |     * *
    \       *       /   /     ISP 2  V  \     |ALTO Client|**** * *
     \      *      /   / +-------------+ \    +-----------+   * * *
      \     *     /    | | ALTO Server | |    +-----------+   * * *
       `-.  *  ,-'     \ +-------------+<====>|   Peer 4  |** * * *
          `-*-'         \               /     |ALTO Client| * * * *
            *            `-.         ,-'      +-----------+ * * * *
            *               `-------'                       * * * *
            *                                               * * * *
       === ALTO protocol
       *** Application protocol

             Figure 21: Global tracker and local ALTO servers

   Figure 21 depicts a tracker-based P2P system with several peers.  The
   peers (i.e., resource consumers) embed an ALTO client to improve the
   resource provider selection.  The tracker (i.e., resource directory)
   itself may be hosted and operated by another entity.  A tracker
   external to the ISPs of the peers may be a typical use case.  For
   instance, a tracker like Pirate Bay can serve BitTorrent peers world-
   wide.  The figure only shows one tracker instance, but deployments
   with several trackers could be possible, too.

   The scenario depicted in Figure 21 lets the peers directly
   communicate with their ISP's ALTO server (i.e., ALTO client embedded
   in the peers), thus giving the peers the most control on which
   information they query for, as they can integrate information
   received from one tracker or several trackers and through direct
   peer-to-peer knowledge exchange.  For instance, the latter approach
   is called peer exchange (PEX) in BitTorrent.  In this deployment
   scenarios, the peers have to discover a suitable ALTO server (e.g.,
   offered by their ISP, as described in [RFC7286]).

   There are also tracker-less P2P system architectures that do not rely
   on centralized resource directories, e.g., unstructured P2P networks.
   Regarding the use of ALTO, their deployment would be similar to

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   Figure 21, since the ALTO client would be embedded in the peers as
   well.  This option is not further considered in this memo.

          ,---.               ,-'         `-.   +-----------+
       ,-'     `-.           /     ISP 1     \  |   Peer 1  |********
      /           \         / +-------------+ \ |           |       *
     /    ISP X    \   ++====>| ALTO Server |  )+-----------+       *
    /               \  ||   \ +-------------+ / +-----------+       *
   ; +-----------+   : ||    \               /  |   Peer 2  |       *
   | |  Tracker  |<====++     `-.         ,-'   |           |****** *
   | |ALTO Client|   |           `-------'      +-----------+     * *
   | +-----------+<====++        ,-------.                        * *
   :        *        ; ||     ,-'         `-.   +-----------+     * *
    \       *       /  ||    /     ISP 2     \  |   Peer 3  |     * *
     \      *      /   ||   / +-------------+ \ |           |**** * *
      \     *     /    ++====>| ALTO Server |  )+-----------+   * * *
       `-.  *  ,-'          \ +-------------+ / +-----------+   * * *
          `-*-'              \               /  |   Peer 4  |** * * *
            *                 `-.         ,-'   |           | * * * *
            *                    `-------'      +-----------+ * * * *
            *                                                 * * * *
            *                                                 * * * *
       === ALTO protocol
       *** Application protocol

      Figure 22: Global tracker accessing ALTO server at various ISPs

   An alternative deployment scenario for a tracker-based system is
   depicted in Figure 22.  Here, the tracker embeds the ALTO client.
   When the tracker receives a request from a querying peer, it first
   discovers the ALTO server responsible for the querying peer.  This
   discovery can be done by using various ALTO server discovery
   mechanisms [RFC7286] [I-D.kiesel-alto-xdom-disc].  The ALTO client
   subsequently sends to the querying peer only those peers that are
   preferred by the ALTO server responsible for the querying peer.  The
   peers do not query the ALTO servers themselves.  This gives the peers
   a better initial selection of candidates, but does not consider peers
   learned through direct peer-to-peer knowledge exchange.

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                      ISP 1  ,-------.         +-----------+
           ,---.          +-------------+******|   Peer 1  |
        ,-'     `-.      /|   Tracker   |\     |           |
       /           \    / +-------------+****  +-----------+
      /    ISP X    \   |       ===       | *  +-----------+
     /               \  \ +-------------+ / *  |   Peer 2  |
    ;   +---------+   :  \| AlTO Server |/  ***|           |
    |   | Global  |   |   +-------------+      +-----------+
    |   | Tracker |   |      `-------'
    |   +---------+   |                        +-----------+
    :        *        ;      ,-------.         |   Peer 3  |
     \       *       /    +-------------+  ****|           |
      \      *      /    /|   Tracker   |***   +-----------+
       \     *     /    / +-------------+ \    +-----------+
        `-.  *  ,-'     |       ===       |    |   Peer 4  |**
           `-*-'        \ +-------------+ /    |           | *
             *           \| ALTO Server |/     +-----------+ *
             *            +-------------+                    *
             *        ISP 2  `-------'                       *
        === ALTO protocol
        *** Application protocol

      Figure 23: Local trackers and local ALTO servers (P4P approach)

   There are some attempts to let ISPs deploy their own trackers, as
   shown in Figure 23.  In this case, the client cannot get guidance
   from the ALTO server other than by talking to the ISP's tracker,
   which in turn communicates with the ALTO server using the ALTO
   protocol.  It should be noted that the peers are still allowed to
   contact other trackers operated by entities other than the peer's
   ISP, but in this case they cannot benefit from ALTO guidance.

4.2.  Deployment Recommendations

4.2.1.  ALTO Services

   The ALTO protocol specification [RFC7285] details how an ALTO client
   can query an ALTO server for guiding information and receive the
   corresponding replies.  In case of peer-to-peer networks, two
   different ALTO services can be used: The Cost Map Service is often
   preferred as solution by peer-to-peer software implementors and
   users, since it avoids disclosing peer IP addresses to a centralized
   entity.  Alternatively, network operators may have a preference for
   the Endpoint Cost Service (ECS), since it does not require exposure
   of the network topology.

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   For actual use of ALTO in P2P applications, both software vendors and
   network operators have to agree which ALTO services to use.  The ALTO
   protocol is flexible and supports both services.  Note that for other
   use cases of ALTO, in particular in more controlled environments,
   both the Cost Map Service as well as Endpoint Cost Service might be
   feasible and it is more an engineering trade-off whether to use a
   map-based or query-based ALTO service.

4.2.2.  Guidance Considerations

   As explained in Section 4.1.2, for a tracker-based P2P application
   there are two fundamentally different possibilities where to place
   the ALTO client:

   1.  ALTO client in the resource consumer ("peer")

   2.  ALTO client in the resource directory ("tracker")

   Both approaches have advantages and drawbacks that have to be
   considered.  If the ALTO client is in the resource consumer
   (Figure 21), a potentially very large number of clients has to be
   deployed.  Instead, when using an ALTO client in the resource
   directory (Figure 22 and Figure 23), ostensibly peers do not have to
   directly query the ALTO server.  In this case, an ALTO server could
   even not permit access to peers.

   However, it seems to be beneficial for all participants to let the
   peers directly query the ALTO server.  Considering the plethora of
   different applications that could use ALTO, e.g. multiple tracker or
   non-tracker based P2P systems or other applications searching for
   relays, this renders the ALTO service more useful.  The peers are
   also the single point having all operational knowledge to decide
   whether to use the ALTO guidance and how to use the ALTO guidance.
   For a given peer one can also expect that an ALTO server of the
   corresponding ISP provides useful guidance and can be discovered.

   Yet, ALTO clients in the resource consumer also have drawbacks
   compared to use in the resource directory.  In the following, both
   scenarios are compared more in detail in order to explain the impact
   on ALTO guidance and the need for third-party ALTO queries.

   In the first scenario (see Figure 24), the peer (resource consumer)
   queries the tracker (resource directory) for the desired resource
   (F1).  The resource directory returns a list of potential resource
   providers without considering ALTO (F2).  It is then the duty of the
   resource consumer to invoke ALTO (F3/F4), in order to solicit
   guidance regarding this list.

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   Peer w. ALTO cli.            Tracker               ALTO Server
   --------+--------       --------+--------       --------+--------
           | F1 Tracker query      |                       |
           |======================>|                       |
           | F2 Tracker reply      |                       |
           |<======================|                       |
           | F3 ALTO protocol query                        |
           | F4 ALTO protocol reply                        |
           |                       |                       |

   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO protocol

      Figure 24: Basic message sequence chart for resource consumer-
                           initiated ALTO query

   In the second scenario (see Figure 25), the resource directory has an
   embedded ALTO client, which we will refer to as Resource Directory
   ALTO Client (RDAC) in this document.  After receiving a query for a
   given resource (F1) the resource directory invokes the RDAC to
   evaluate all resource providers it knows (F2/F3).  Then it returns a,
   possibly shortened, list containing the "best" resource providers to
   the resource consumer (F4).

         Peer               Tracker w. RDAC           ALTO Server
   --------+--------       --------+--------       --------+--------
           | F1 Tracker query      |                       |
           |======================>|                       |
           |                       | F2 ALTO cli. p. query |
           |                       |---------------------->|
           |                       | F3 ALTO cli. p. reply |
           |                       |<----------------------|
           | F4 Tracker reply      |                       |
           |<======================|                       |
           |                       |                       |

   ====  Application protocol (i.e., tracker-based P2P app protocol)
   ----  ALTO protocol

    Figure 25: Basic message sequence chart for third-party ALTO query

   Note: The message sequences depicted in Figure 24 and Figure 25 may
   occur both in the target-aware and the target-independent query mode
   (cf. [RFC6708]).  In the target-independent query mode no message
   exchange with the ALTO server might be needed after the tracker

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   query, because the candidate resource providers could be evaluated
   using a locally cached "map", which has been retrieved from the ALTO
   server some time ago.

   The first approach has the following problem: While the resource
   directory might know thousands of peers taking part in a swarm, the
   list returned to the resource consumer is usually shortened for
   efficiency reasons.  Therefore, the "best" (in the sense of ALTO)
   potential resource providers might not be contained in that list
   anymore, even before ALTO can consider them.

   Much better traffic optimization could be achieved if the tracker
   would evaluate all known peers using ALTO.  This list would then
   include a significantly higher fraction of "good" peers.  If the
   tracker returned "good" peers only, there might be a risk that the
   swarm might disconnect and split into several disjunct partitions.
   However, finding the right mix of ALTO-biased and random peer
   selection is out of the scope of this document.

   Therefore, from an overall optimization perspective, the second
   scenario with the ALTO client embedded in the resource directory is
   advantageous, because it is ensured that the addresses of the "best"
   resource providers are actually delivered to the resource consumer.
   An architectural implication of this insight is that the ALTO server
   discovery procedures must support third-party discovery.  That is, as
   the tracker issues ALTO queries on behalf of the peer which contacted
   the tracker, the tracker must be able to discover an ALTO server that
   can give guidance suitable for that respective peer (see

   In principle, a combined approach could also be possible.  For
   instance, a tracker could use a coarse-grained "global" ALTO server
   to find the peers in the general vicinity of the requesting peer,
   while peers could use "local" ALTO servers for a more fine-grained
   guidance.  Yet, there is no known deployment experience for such a
   combined approach.

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5.  Using ALTO for CDNs

5.1.  Overview

5.1.1.  Usage Scenario

   This section briefly introduces the usage of ALTO for Content
   Delivery Networks (CDNs), as explained in
   [I-D.jenkins-alto-cdn-use-cases].  CDNs are used in the delivery of
   some Internet services (e.g., delivery of websites, software updates
   and video delivery) from a location closer to the location of the
   user.  A CDN typically consists of a network of servers often
   attached to Internet Service Provider (ISP) networks.  The point of
   attachment is often as close to content consumers and peering points
   as economically or operationally feasible in order to decrease
   traffic load on the ISP backbone and to provide better user
   experience measured by reduced latency and higher throughput.

   CDNs use several techniques to redirect a client to a server
   (surrogate).  A request routing function within a CDN is responsible
   for receiving content requests from user agents, obtaining and
   maintaining necessary information about a set of candidate
   surrogates, and for selecting and redirecting the user agent to the
   appropriate surrogate.  One common way is relying on the DNS system,
   but there are many other ways, see [RFC3568].

   | CDN Request Router |
   |  with ALTO Client  |
             || ALTO protocol
         |  ALTO   |
         | Server  |
              : Provisioning protocol
     ,-'  Source of  `-.
    (    topological    )
     `-. information ,-'

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        Figure 26: Use of ALTO information for CDN request routing

   In order to derive the optimal benefit from a CDN it is preferable to
   deliver content from the servers (caches) that are "closest" to the
   end user requesting the content. "closest" may be as simple as
   geographical or IP topology distance, but it may also consider other
   combinations of metrics and CDN or Internet Service Provider (ISP)
   policies.  As illustrated in Figure 26, ALTO could provide this

   User Agent                  Request Router                 Surrogate
        |                             |                           |
        |     F1 Initial Request      |                           |
        +---------------------------->|                           |
        |                             +--+                        |
        |                             |  | F2 Surrogate Selection |
        |                             |<-+       (using ALTO)     |
        |   F3 Redirection Response   |                           |
        |<----------------------------+                           |
        |                             |                           |
        |     F4 Content Request      |                           |
        |                             |                           |
        |                             |          F5 Content       |
        |                             |                           |

               Figure 27: Example of CDN surrogate selection

   Figure 27 illustrates the interaction between a user agent, a request
   router, and a surrogate for the delivery of content in a single CDN.
   As explained in [I-D.jenkins-alto-cdn-use-cases], the user agent
   makes an initial request to the CDN (F1).  This may be an
   application-level request (e.g., HTTP) or a DNS request.  In the
   second step (F2), the request router selects an appropriate surrogate
   (or set of surrogates) based on the user agent's (or its proxy's) IP
   address, the request router's knowledge of the network topology
   (which can be obtained by ALTO) and reachability cost between CDN
   caches and end users, and any additional CDN policies.  Then (F3),
   the request router responds to the initial request with an
   appropriate response containing a redirection to the selected cache,
   for example by returning an appropriate DNS A/AAAA record, a HTTP 302
   redirect, etc.  The user agent uses this information to connect
   directly to the surrogate and request the desired content (F4), which
   is then delivered (F5).

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5.1.2.  Applicability of ALTO

   The most simple use case for ALTO in a CDN context is to improve the
   selection of a CDN surrogate or origin.  In this case, the CDN makes
   use of an ALTO server to choose a better CDN surrogate or origin than
   would otherwise be the case.  Although it is possible to obtain raw
   network map and cost information in other ways, for example passively
   listening to the ISP's routing protocols or use of active probing,
   the use of an ALTO service to expose that information may provide
   additional control to the ISP over how their network map/cost is
   exposed.  Additionally it may enable the ISP to maintain a functional
   separation between their routing plane and network map computation
   functions.  This may be attractive for a number of reasons, for

   o  The ALTO service could provide a filtered view of the network
      and/or cost map that relates to CDN locations and their proximity
      to end users, for example to allow the ISP to control the level of
      topology detail they are willing to share with the CDN.

   o  The ALTO service could apply additional policies to the network
      map and cost information to provide a CDN-specific view of the
      network map/cost, for example to allow the ISP to encourage the
      CDN to use network links that would not ordinarily be preferred by
      a Shortest Path First routing calculation.

   o  The routing plane may be operated and controlled by a different
      operational entity (even within a single ISP) than the CDN.
      Therefore, the CDN may not be able to passively listen to routing
      protocols, nor may it have access to other network topology data
      (e.g., inventory databases).

   When CDN servers are deployed outside of an ISP's network or in a
   small number of central locations within an ISP's network, a
   simplified view of the ISP's topology or an approximation of
   proximity is typically sufficient to enable the CDN to serve end
   users from the optimal server/location.  As CDN servers are deployed
   deeper within ISP networks it becomes necessary for the CDN to have
   more detailed knowledge of the underlying network topology and costs
   between network locations in order to enable the CDN to serve end
   users from the optimal servers for the ISP.

   The request router in a CDN will typically also take into account
   criteria and constraints that are not related to network topology,
   such as the current load of CDN surrogates, content owner policies,
   end user subscriptions, etc.  This document only discusses use of
   ALTO for network information.

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   A general issue for CDNs is that the CDN logic has to match the
   client's IP address with the closest CDN surrogate, both for DNS or
   HTTP redirect based approaches (see, for instance,
   [I-D.penno-alto-cdn]).  This matching is not trivial, for instance,
   in DNS based approaches, where the IP address of the DNS original
   requester is unknown (see [I-D.ietf-dnsop-edns-client-subnet] for a
   discussion of this and a solution approach).

   In addition to use by a single CDN, ALTO can also be used in
   scenarios that interconnect several CDNs.  This use case is detailed
   in [I-D.seedorf-cdni-request-routing-alto].

5.2.  Deployment Recommendations

5.2.1.  ALTO Services

   In its simplest form an ALTO server would provide an ISP with the
   capability to offer a service to a CDN that provides network map and
   cost information.  The CDN can use that data to enhance its surrogate
   and/or origin selection.  If an ISP offers an ALTO network and cost
   map service to expose a cost mapping/ranking between end user IP
   subnets (within that ISP's network) and CDN surrogate IP subnets/
   locations, periodic updates of the maps may be needed.  As introduced
   in Section 3.3), it is common for broadband subscribers to obtain
   their IP addresses dynamically and in many deployments the IP subnets
   allocated to a particular network region can change relatively
   frequently, even if the network topology itself is reasonably static.

   An alternative would be to use the ALTO Endpoint Cost Service (ECS):
   When an end user requests a given content, the CDN request router
   issues an ECS request with the endpoint address (IPv4/IPv6) of the
   end user (content requester) and the set of endpoint addresses of the
   surrogate (content targets).  The ALTO server receives the request
   and ranks the addresses based on their distance from the content
   requester.  Once the request router obtained from the ALTO server the
   ranked list of locations (for the specific user), it can incorporate
   this information into its selection mechanisms in order to point the
   user to the most appropriate surrogate.

   Since CDNs operate in a controlled environment, the ALTO network/cost
   map service and ECS have a similar level of security and
   confidentiality of network-internal information.  However, the
   network/cost map service and ECS differ in the way the ALTO service
   is delivered and address a different set of requirements in terms of
   topology information and network operations.

   If a CDN already has means to model connectivity policies, the map-
   based approaches could possibly be integrated into that.  If the ECS

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   service is preferred, a request router that uses ECS could cache the
   results of ECS queries for later usage in order to address the
   scalability limitations of ECS and to reduce the number of
   transactions between CDN and ALTO server.  The ALTO server may
   indicate in the reply message how long the content of the message is
   to be considered reliable and insert a lifetime value that will be
   used by the CDN in order to cache (and then flush or refresh) the

5.2.2.  Guidance Considerations

   In the following it is discussed how a CDN could make use of ALTO

   In one deployment scenario, ALTO could expose ISP end user
   reachability to a CDN.  The request router needs to have information
   about which end user IP subnets are reachable via which networks or
   network locations.  The network map services offered by ALTO could be
   used to expose this topology information while avoiding routing plane
   peering between the ISP and the CDN.  For example, if CDN surrogates
   are deployed within the access or aggregation network, the ISP is
   likely to want to utilize the surrogates deployed in the same access/
   aggregation region in preference to surrogates deployed elsewhere, in
   order to alleviate the cost and/or improve the user experience.

   In addition, CDN surrogates could also use ALTO guidance, e.g., if
   there is more than one upstream source of content or several origins.
   In this case, ALTO could help a surrogate with the decision about
   which upstream source to use.  This specific variant of using ALTO is
   not further detailed in this document.

   If content can be provided by several CDNs, there may be a need to
   interconnect these CDNs.  In this case, ALTO can be uses as an
   interface [I-D.seedorf-cdni-request-routing-alto], in particular for
   footprint and capabilities advertisement.

   Other and more advanced scenarios of deploying ALTO are also listed
   in [I-D.jenkins-alto-cdn-use-cases] and [I-D.penno-alto-cdn].

   The granularity of ALTO information required depends on the specific
   deployment of the CDN.  For example, an "over-the-top" CDN whose
   surrogates are deployed only within the Internet backbone may only
   require knowledge of which end user IP subnets are reachable via
   which ISPs' networks, whereas a CDN deployed within a particular
   ISP's network requires a finer granularity of knowledge.

   An ALTO server ranks addresses based on topology information it
   acquires from the network.  By default, according to [RFC7285],

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   distance in ALTO represents an abstract "routingcost" that can be
   computed for instance from routing protocol information.  But an ALTO
   server may also take into consideration other criteria or other
   information sources for policy, state, and performance information
   (e.g., geo-location), as explained in Section 3.2.2.

   The different methods and algorithms through which the ALTO server
   computes topology information and rankings is out of the scope of
   this document.  If rankings are based on routing protocol
   information, it is obvious that network events may impact the ranking
   computation.  Due to internal redundancy and resilience mechanisms
   inside current networks, most of the network events happening in the
   infrastructure will be handled internally in the network, and they
   should have limited impact on a CDN.  However, catastrophic events
   such as main trunks failures or backbone partitioning will have to be
   taken into account by the ALTO server to redirect traffic away from
   the impacted area.

   An ALTO server implementation may want to keep state about ALTO
   clients so to inform and signal to these clients when a major network
   event happened, e.g., by a notification mechanism.  In a CDN/ALTO
   interworking architecture with few CDN components interacting with
   the ALTO server there are less scalability issues in maintaining
   state about clients in the ALTO server, compared to ALTO guidance to
   any Internet user.

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6.  Other Use Cases

   This section briefly surveys and references other use cases that have
   been tested or suggested for ALTO deployments.

6.1.  Application Guidance in Virtual Private Networks (VPNs)

   Virtual Private Network (VPN) technology is widely used in public and
   private networks to create groups of users that are separated from
   other users of the network and allows these users to communicate
   among themselves as if they were on a private network.  Network
   Service Providers (NSPs) offer different types of VPNs.  [RFC4026]
   distinguishes between Layer 2 VPN (L2VPN) and Layer 3 VPN (L3VPN)
   using different sub-types.  In the following, the term "VPN" is used
   to refer to provider supplied virtual private networking.

   From the perspective of an application at an endpoint, a VPN may not
   be very different to any other IP connectivity solution, but there
   are a number of specific applications that could benefit from ALTO
   topology exposure and guidance in VPNs.  As in the general Internet,
   one advantage is that applications do not have to perform excessive
   measurements on their own.  For instance, potential use cases for
   ALTO application guidance in VPN environments are:

   o  Enterprise application optimization: Enterprise customers often
      run distributed applications that exchange large amounts of data,
      e.g., for synchronization of replicated data bases.  Network
      topology information could be useful for placement of replicas as
      well as for the scheduling of transfers.

   o  Private cloud computing solution: An enterprise customer could run
      its own data centers at the four sites.  The cloud management
      system could want to understand the network costs between
      different sites for intelligent routing and placement decisions of
      Virtual Machines (VMs) among the VPN sites.

   o  Cloud-bursting: One or more VPN endpoints could be located in a
      public cloud.  If an enterprise customer needs additional
      resources, they could be provided by a public cloud, which is
      accessed through the VPN.  Network topology awareness would help
      to decide in which data center of the public cloud those resources
      should be allocated.

   These examples focus on enterprises, which are typical users of VPNs.
   VPN customers typically have no insight into the network topology
   that transports the VPN.  Similar to other ALTO use cases, better-
   than-random application-level decisions would be enabled by an ALTO
   server offered by the NSP, as illustrated in Figure 28.

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                       |  Customer's   |
                       |   management  |
                       |  application  |.
                       | (ALTO client) |  .
                       +---------------+    .  VPN provisioning
                              /\              . (out-of-scope)
                              || ALTO           .
                              \/                  .
                    +---------------------+       +----------------+
                    |     ALTO server     |       | VPN portal/OSS |
                    |   provided by NSP   |       | (out-of-scope) |
                    +---------------------+       +----------------+
                               : VPN network
                               : and cost maps
                     /---------:---------\ Network service provider
                     |         :         |
        +-------+   _______________________   +-------+
        | App a | ()_____. .________. .____() | App d |
        +-------+    |   | |        | |  |    +-------+
                     \---| |--------| |--/
                         | |        | |
                         |^|        |^| Customer VPN
                          V          V
                      +-------+  +-------+
                      | App b |  | App c |
                      +-------+  +-------+

                       Figure 28: Using ALTO in VPNs

   A common characteristic of these use cases is that applications will
   not necessarily run in the public Internet, and that the relationship
   between the provider and customer of the VPN is rather well-defined.
   Since VPNs often run in a managed environment, an ALTO server may
   have access to topology information (e.g., traffic engineering data)
   that would not be available for the public Internet, and it may
   expose it to the customer of the VPN only.

   Also, a VPN will not necessarily be static.  The customer could
   possibly modify the VPN and add new VPN sites by a Web portal,
   network management systems, or other Operation Support Systems (OSS)
   solutions.  Prior to adding a new VPN site, an application will not
   have connectivity to that site, i.e., an ALTO server could offer
   access to information that an application cannot measure on its own
   (e.g., expected delay to a new VPN site).

   The VPN use cases, requirements, and solutions are further detailed

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   in [I-D.scharf-alto-vpn-service].

6.2.  In-Network Caching

   Deployment of intra-domain P2P caches has been proposed for
   cooperation between the network operator and the P2P service
   providers, e.g., to reduce the bandwidth consumption in access
   networks [I-D.deng-alto-p2pcache].

            +--------------+                +------+
            | ISP 1 network+----------------+Peer 1|
            +-----+--------+                +------+
   |        |                                      ISP 2 network   |
   |  +---------+                                                  |
   |  |L1 Cache |                                                  |
   |  +-----+---+                                                  |
   |        +--------------------+----------------------+          |
   |        |                    |                      |          |
   | +------+------+      +------+-------+       +------+-------+  |
   | | AN1         |      | AN2          |       | AN3          |  |
   | | +---------+ |      | +----------+ |       |              |  |
   | | |L2 Cache | |      | |L2 Cache  | |       |              |  |
   | | +---------+ |      | +----------+ |       |              |  |
   | +------+------+      +------+-------+       +------+-------+  |
   |        |                                           |          |
   |        +--------------------+                      |          |
   |        |                    |                      |          |
   | +------+------+      +------+-------+       +------+-------+  |
   | | SUB-AN11    |      | SUB-AN12     |       | SUB-AN31     |  |
   | | +---------+ |      |              |       |              |  |
   | | |L3 Cache | |      |              |       |              |  |
   | | +---------+ |      |              |       |              |  |
   | +------+------+      +------+-------+       +------+-------+  |
   |        |                    |                      |          |
            |                    |                      |
        +---+---+            +---+---+                  |
        |       |            |       |                  |
     +--+--+ +--+--+      +--+--+ +--+--+            +--+--+
     |Peer2| |Peer3|      |Peer4| |Peer5|            |Peer6|
     +-----+ +-----+      +-----+ +-----+            +-----+

            Figure 29: General architecture of intra-ISP caches

   Figure 29 depicts the overall architecture of potential P2P cache

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   deployments inside an ISP 2 with various access network types.  As
   shown in the figure, P2P caches may be deployed at various levels,
   including the interworking gateway linking with other ISPs, internal
   access network gateways linking with different types of accessing
   networks (e.g.  WLAN, cellular and wired), and even within an
   accessing network at the entries of individual WLAN sub-networks.
   Moreover, depending on the network context and the operator's policy,
   each cache can be a Forwarding Cache or a Bidirectional Cache

   In such a cache architecture, the locations of caches could be used
   as dividers of different PIDs to guide intra-ISP network abstraction
   and mark costs among them according to the location and type of
   relevant caches.

   Further details and deployment considerations can be found in

6.3.  Other Application-based Network Operations

   An ALTO server can be part of an overall framework for Application-
   Based Network Operations (ABNO) [RFC7491] that brings together
   different technologies for gathering information about the resources
   available in a network, for consideration of topologies and how those
   topologies map to underlying network resources, for requesting path
   computation, and for provisioning or reserving network resources.
   Such an architecture may include additional components such as a Path
   Computation Element (PCE) for on-demand and application-specific
   reservation of network connectivity, reliability, and resources (such
   as bandwidth).  Some use cases how to leverage ALTO for joint network
   and application-layer optimization are explained in [RFC7491].

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7.  Security Considerations

   Security concerns were extensively discussed from the very beginning
   of the development of the ALTO protocol, and they have been
   considered in detail in the ALTO requirements document [RFC6708] as
   well as in the ALTO protocol specification document [RFC7285].  The
   two main security concerns are related to the unwanted disclosure of
   information through ALTO and the negative impact of specially
   crafted, wrong ("faked") guidance presented to an ALTO client.  In
   addition to this, the usual concerns related to the operation of any
   networked application apply.

   This section focuses on the peer-to-peer use case, which is - from a
   security perspective - probably the most difficult ALTO use case that
   has been considered.  Special attention is given to the two main
   security concerns.

7.1.  ALTO as a Protocol Crossing Trust Boundaries

   The optimization of peer-to-peer applications was the first use case
   and the impetus for the development of the ALTO protocol, in
   particular file sharing applications such as BitTorrent [RFC5594].

   As explained in Section 4.1.1, for the publisher of the ALTO
   information (i.e., the ALTO server operator) it may not be apparent
   who is in charge of the P2P application overlay.  Some P2P
   applications do not have any central control entity and the whole
   overlay consists only of the peers, which are under control of the
   individual users.  Other P2P applications may have some control
   entities such as super peers or trackers, but these may be located in
   foreign countries and under the control of unknown organizations.  As
   outlined in Section 4.2.2, in some scenarios it may be very
   beneficial to forward ALTO information to such trackers, super peers,
   etc. located in remote networks.  This situation is aggravated by the
   vast number of different P2P applications which are evolving quickly
   and often without any coordination with the network operators.

   In summary it can be said that in many instances of the P2P use case,
   the ALTO protocol bridges the border between the "managed" IP network
   infrastructure under strict administrative control and one or more
   "unmanaged" application overlays, i.e., overlays for which it is hard
   to tell who is in charge of them.  This differs from more controlled
   environments (e.g., in the CDN use case), in which bilateral
   agreements between the producer and consumer of guidance are

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7.2.  Information Leakage from the ALTO Server

   An ALTO server will be provisioned with information about the ISP's
   network and possibly also with information about neighboring ISPs.
   This information (e.g., network topology, business relations, etc.)
   is often considered to be confidential to the ISP and can include
   very sensitive information.  ALTO does not require any particular
   level of details of information disclosure, and hence the provider
   should evaluate how much information is revealed and the associated

   Furthermore, if the ALTO information is very fine grained, it may
   also be considered sensitive with respect to user privacy.  For
   example, consider a hypothetical endpoint property "provisioned
   access link bandwidth" or "access technology (ADSL, VDSL, FTTH,
   etc.)" and an ALTO service that publishes this property for
   individual IP addresses.  This information could not only be used for
   traffic optimization but, for example, also for targeted advertising
   to residential users with exceptionally good (or bad) connectivity,
   such as special banner ads.  For an advertisement system it would be
   more complex to obtain such information otherwise, e.g., by bandwidth

   Different scenarios related to the unwanted disclosure of an ALTO
   server's information have been itemized and categorized in RFC 6708,
   Section 5.2.1., cases (1)-(3) [RFC6708].

   In some use cases it is not possible to use access control (see
   Section 7.3) to limit the distribution of ALTO knowledge to a small
   set of trusted clients.  In these scenarios it seems tempting not to
   use network maps and cost maps at all, and instead completely rely on
   endpoint cost service and endpoint ranking in the ALTO server.  While
   this practice may indeed reduce the amount of information that is
   disclosed to an individual ALTO client, some issues should be
   considered: First, when using the map based approach, it is trivial
   to analyze the maximum amount of information that could be disclosed
   to a client: the full maps.  In contrast, when providing endpoint
   cost service only, the ALTO server operator could be prone to a false
   feeling of security, while clients use repeated queries and/or
   collaboration to gather more information than they are expected to
   get (see Section 5.2.1., case (3) in [RFC6708]).  Second, the
   endpoint cost service reveals more information about the user or
   application behavior to the ALTO server, e.g., which other hosts are
   considered as peers for the exchange of a significant amount of data
   (see Section 5.2.1., cases (4)-(6) in [RFC6708]).

   Consequently, users may be more reluctant to use the ALTO service at
   all if it is based on the endpoint cost service instead of providing

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   network and cost maps.  Given that some popular P2P applications are
   sometimes used for purposes such as distribution of files without the
   explicit permission from the copyright owner, it may also be in the
   interest of the ALTO server operator that an ALTO server cannot infer
   the behavior of the application to be optimized.  One possible
   conclusion could be to publish network and cost maps through ALTO
   that are so coarse-grained that they do not violate the network
   operator's or the user's interests.

   In other use cases in more controlled environments (e.g., in the CDN
   use case) bilateral agreements, access control (see Section 7.3), and
   encryption could be used to reduce the risk of information leakage.

7.3.  ALTO Server Access

   Depending on the use case of ALTO, it may be desired to apply access
   restrictions to an ALTO server, i.e., by requiring client
   authentication.  According to [RFC7285], ALTO requires that HTTP
   Digest Authentication is supported, in order to achieve client
   authentication and possibly to limit the number of parties with whom
   ALTO information is directly shared.  TLS Client Authentication may
   also be supported.

   In general, well-known security management techniques and best
   current practices [RFC4778] for operational ISP infrastructure also
   apply to an ALTO service, including functions to protect the system
   from unauthorized access, key management, reporting security-relevant
   events, and authorizing user access and privileges.

   For peer-to-peer applications, a potential deployment scenario is
   that an ALTO server is solely accessible by peers from the ISP
   network (as shown in Figure 21).  For instance, the source IP address
   can be used to grant only access from that ISP network to the server.
   This will "limit" the number of peers able to attack the server to
   the user's of the ISP (however, including compromised computers that
   are part of a botnet).

   If the ALTO server has to be accessible by parties not located in the
   ISP's network (see Figure 22), e.g., by a third-party tracker or by a
   CDN system outside the ISP's network, the access restrictions have to
   be looser.  In the extreme case, i.e., no access restrictions, each
   and every host in the Internet can access the ALTO server.  This
   might no be the intention of the ISP, as the server is not only
   subject to more possible attacks, but also the server load could
   increase, since possibly more ALTO clients have to be served.

   There are also use cases where the access to the ALTO server has to
   be much more strictly controlled, i. e., where an authentication and

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   authorization of the ALTO client to the server may be needed.  For
   instance, in case of CDN optimization the provider of an ALTO service
   as well as potential users are possibly well-known.  Only CDN
   entities may need ALTO access; access to the ALTO servers by
   residential users may neither be necessary nor be desired.

   Access control can also help to prevent Denial-of-Service attacks by
   arbitrary hosts from the Internet.  Denial-of-Service (DoS) can both
   affect an ALTO server and an ALTO client.  A server can get
   overloaded if too many requests hit the server, or if the query load
   of the server surpasses the maximum computing capacity.  An ALTO
   client can get overloaded if the responses from the sever are, either
   intentionally or due to an implementation mistake, too large to be
   handled by that particular client.

7.4.  Faking ALTO Guidance

   The ALTO services enables an ALTO service provider to influence the
   behavior of network applications.  An attacker who is able to
   generate false replies, or e.g. an attacker who can intercept the
   ALTO server discovery procedure, can provide faked ALTO guidance.

   Here is a list of examples how the ALTO guidance could be faked and
   what possible consequences may arise:

   Sorting:  An attacker could change the sorting order of the ALTO
      guidance (given that the order is of importance, otherwise the
      ranking mechanism is of interest), i.e., declaring peers located
      outside the ISP as peers to be preferred.  This will not pose a
      big risk to the network or peers, as it would mimic the "regular"
      peer operation without traffic localization, apart from the
      communication/processing overhead for ALTO.  However, it could
      mean that ALTO is reaching the opposite goal of shuffling more
      data across ISP boundaries, incurring more costs for the ISP.  In
      another example, fake guidance could give unrealistically low
      costs to devices in an ISP's mobile network, thus encouraging
      other devices to contact them, thereby degrading the ISP's mobile
      network and causing customer dissatisfaction.

   Preference of a single peer:  A single IP address (thus a peer) could
      be marked as to be preferred all over other peers.  This peer can
      be located within the local ISP or also in other parts of the
      Internet (e.g., a web server).  This could lead to the case that
      quite a number of peers to trying to contact this IP address,
      possibly causing a Denial-of-Service (DoS) attack.

   The ALTO protocol protects the authenticity and integrity of ALTO
   information while in transit by leveraging the authenticity and

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   integrity protection mechanisms in TLS (see Section 8.3.5 of
   [RFC7285]).  It has not yet been investigated how wrong ALTO guidance
   given by an autheticated ALTO server can impact the operation of the
   network and the applications.

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8.  IANA Considerations

   This document makes no specific request to IANA.

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9.  Acknowledgments

   This memo is the result of contributions made by several people:

   o  Xianghue Sun, Lee Kai, and Richard Yang contributed text on ISP
      deployment requirements and monitoring.

   o  Stefano Previdi contributed parts of the Section 5 on "Using ALTO
      for CDNs".

   o  Rich Woundy contributed text to Section 3.3.

   o  Lingli Deng, Wei Chen, Qiuchao Yi, and Yan Zhang contributed
      Section 6.2.

   Thomas-Rolf Banniza, Vinayak Hegde, Qin Wu, Wendy Roome, and Sabine
   Randriamasy provided very useful comments and reviewed the document.

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10.  References

10.1.  Normative References

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693,
              October 2009.

   [RFC6708]  Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
              Y. Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", RFC 6708, September 2012.

   [RFC7285]  Alimi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S.,
              Roome, W., Shalunov, S., and R. Woundy, "Application-Layer
              Traffic Optimization (ALTO) Protocol", RFC 7285,
              September 2014.

   [RFC7286]  Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
              H. Song, "Application-Layer Traffic Optimization (ALTO)
              Server Discovery", RFC 7286, November 2014.

10.2.  Informative References

              Lingli, D., Chen, W., Yi, Q., and Y. Zhang,
              "Considerations for ALTO with network-deployed P2P
              caches", draft-deng-alto-p2pcache-03 (work in progress),
              February 2014.

              Roome, W. and Y. Yang, "ALTO Incremental Updates Using
              Server-Sent Events (SSE)",
              draft-ietf-alto-incr-update-sse-02 (work in progress),
              April 2016.

              Contavalli, C., Gaast, W., tale, t., and W. Kumari,
              "Client Subnet in DNS Queries",
              draft-ietf-dnsop-edns-client-subnet-07 (work in progress),
              March 2016.

              Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
              Nadeau, "An Architecture for the Interface to the Routing
              System", draft-ietf-i2rs-architecture-13 (work in
              progress), February 2016.


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              Clemm, A., Medved, J., Varga, R., Tkacik, T., Bahadur, N.,
              Ananthakrishnan, H., and X. Liu, "A Data Model for Network
              Topologies", draft-ietf-i2rs-yang-network-topo-04 (work in
              progress), July 2016.

              Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
              S. Previdi, "Use Cases for ALTO within CDNs",
              draft-jenkins-alto-cdn-use-cases-03 (work in progress),
              June 2012.

              Kiesel, S. and M. Stiemerling, "ALTO H12",
              draft-kiesel-alto-h12-02 (work in progress), March 2010.

              Kiesel, S. and M. Stiemerling, "Application Layer Traffic
              Optimization (ALTO) Cross-Domain Server Discovery",
              draft-kiesel-alto-xdom-disc-01 (work in progress),
              July 2015.

              Li, K. and G. Jian, "ALTO and DECADE service trial within
              China Telecom", draft-lee-alto-chinatelecom-trial-04 (work
              in progress), March 2012.

              Penno, R., Medved, J., Alimi, R., Yang, R., and S.
              Previdi, "ALTO and Content Delivery Networks",
              draft-penno-alto-cdn-03 (work in progress), March 2011.

              Scharf, M., Gurbani, V., Soprovich, G., and V. Hilt, "The
              Virtual Private Network (VPN) Service in ALTO: Use Cases,
              Requirements and Extensions",
              draft-scharf-alto-vpn-service-02 (work in progress),
              February 2014.

              Seedorf, J., Yang, Y., and J. Peterson, "CDNI Footprint
              and Capabilities Advertisement using ALTO",
              draft-seedorf-cdni-request-routing-alto-08 (work in
              progress), March 2015.

              Seidel, H., "ALTO map calculation from live network data",
              draft-seidel-alto-map-calculation-00 (work in progress),
              October 2015.

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              Wu, Q., Yang, Y., Lee, Y., Dhody, D., and S. Randriamasy,
              "ALTO Traffic Engineering Cost Metrics",
              draft-wu-alto-te-metrics-07 (work in progress),
              March 2016.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [RFC3568]  Barbir, A., Cain, B., Nair, R., and O. Spatscheck, "Known
              Content Network (CN) Request-Routing Mechanisms",
              RFC 3568, July 2003.

   [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
              Private Network (VPN) Terminology", RFC 4026, March 2005.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/
              RFC4655, August 2006,

   [RFC4778]  Kaeo, M., "Operational Security Current Practices in
              Internet Service Provider Environments", RFC 4778,
              January 2007.

   [RFC5594]  Peterson, J. and A. Cooper, "Report from the IETF Workshop
              on Peer-to-Peer (P2P) Infrastructure, May 28, 2008",
              RFC 5594, July 2009.

   [RFC5632]  Griffiths, C., Livingood, J., Popkin, L., Woundy, R., and
              Y. Yang, "Comcast's ISP Experiences in a Proactive Network
              Provider Participation for P2P (P4P) Technical Trial",
              RFC 5632, September 2009.

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)",
              RFC 6241, June 2011.

   [RFC6875]  Kamei, S., Momose, T., Inoue, T., and T. Nishitani, "The
              P2P Network Experiment Council's Activities and
              Experiments with Application-Layer Traffic Optimization
              (ALTO) in Japan", RFC 6875, DOI 10.17487/RFC6875,

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              February 2013, <>.

   [RFC7491]  King, D. and A. Farrel, "A PCE-Based Architecture for
              Application-Based Network Operations", RFC 7491,
              DOI 10.17487/RFC7491, March 2015,

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,

   [RFC7922]  Clarke, J., Salgueiro, G., and C. Pignataro, "Interface to
              the Routing System (I2RS) Traceability: Framework and
              Information Model", RFC 7922, DOI 10.17487/RFC7922,
              June 2016, <>.

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Authors' Addresses

   Martin Stiemerling
   Hochschule Darmstadt


   Sebastian Kiesel
   University of Stuttgart Information Center
   Networks and Communication Systems Department
   Allmandring 30
   Stuttgart  70550


   Michael Scharf
   Lorenzstrasse 10
   Stuttgart  70435


   Hans Seidel


   Stefano Previdi
   Cisco Systems, Inc.
   Via Del Serafico 200
   Rome  00191


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